In films, when a couple meet, they just know that each has found that one special person, and that marriage, children and eternal bliss are just a heartbeat away.
Unfortunately, in the real world, it is much harder to work out whether a relationship has a future or not. Most of us don't have these blinding flashes - or, if we do, we have been let down by them in the past and no longer trust our own judgment.
So we follow the standard advice: get to know each other, go on dates, and ask friends to check him out.
However, there comes a point when you need to decide if this relationship is serious or wasting your time. Is he into you enough - and are you into him enough - to go all the way?
As a marital therapist who has spent 20 years sorting out love dilemmas, I've watched hundreds of couples trying to decide whether their relationship is worth pursuing further. They all approach the problem like a judge trying to weigh up the pros and cons.
But how do you balance present issues and past experience against an unknowable future?
Should 'but I love him' trump infuriating behaviour? No wonder these people find themselves swinging first one way and then the other. But it is possible, normally about six months in, to assess the prospects for your relationship and come to a realistic conclusion.
The secret is to approach it like a doctor, and diagnose its condition by putting it through a series of tests. If it passes them all, your relationship has a clean bill of health and every chance of a successful future.
Here are my seven essential tests...
1. How good are the good times?
Sometimes when people look back over their relationship, it was not as brilliant as they first thought. 'It seemed so good on paper - great restaurant, handsome guy - but I wonder how much my hopes created the spark,' said Alannah, 32, when she recalled her first dates with David, 36.
'Did my fantasies also inflate OK sex into great sex?' Take off the rose-coloured glasses: if the good times still seem good, your relationship passes the first test.
2. Are you more impressed by what he says rather than what he does?
This was the clincher for Alannah. 'He says that he loves me, he can see a future for us,' she explained, 'but he gets really vague when I want to plan something even a couple of months ahead.'
He had failed the test because although he talked about commitment, he did not act committed.
Conversely, for Miranda, 29, this test taught her to trust her new boyfriend. 'He had always been reticent with his feelings so I thought he didn't really care but then I was ill and he arrived at my home with a saucepan of home-made soup.' He passed with flying colours.
3. Is he genuinely interested in what you're doing?
'I would listen to all the intricacies of his latest business deals,' said Emily, 36, 'but he couldn't even remember my best friend's name.'
On her next date, she monitored how much time he talked about himself and his reaction when the conversation turned to her.
'What shocked me was how his eyes glazed over if I talked about something that interested me.'
Emily rightly concluded that he was not into her enough to make this a viable long-term relationship.
4. How does he make you feel about yourself?
The right partner can improve your self-respect, whereas the wrong one can gradually erode your self-belief. If you contradict him, does he make you feel stupid?
Julia, 48, ran her own successful small business but had left school at 16. Her boyfriend Mark, 49, had been to a leading university.
'If something intellectual comes up - some writer or composer - he has this habit of explaining who he is and what he's done. It makes me feel stupid.'
Julia found this especially hurtful when he did it in front of friends. But although a negative response to this question is a black mark against the relationship, there is still hope: see the next question.
5. Can you talk about anything and everything?
Julia talked to Mark about his intellectual name-dropping: 'I told him how belittled I felt by it, and to my surprise, he does it to include me in the conversation.'
They agreed he wouldn't do it any more unless Julia asked. The more subjects that can be discussed openly, the healthier the relationship will be.
Rebecca, another client, confronted this question and realised there was something her boyfriend refused to discuss: 'I knew he wanted to go back to Australia, but he would just evade the topic.'
So Rebecca ended the relationship - at which point her boyfriend admitted he had already booked a solo flight home. Six months into a relationship, you should be able to raise topics such as having children and whether your life ambitions match.
6. Do you both look forward to touching each other?
This one focuses on whether you are into him enough. 'My boyfriend is really nice, he treats me well and he's a good listener too,' said Alison, 28.
'But I have to admit there's no real chemistry.' She stopped for a moment.
'If we're alone for too long he can really get on my nerves.'
It turned out that Alison liked his family - especially his mother and sister - more than him. It is important to have good physical chemistry. Alison ended the relationship and she has not regretted her decision.
7. Is this relationship different from your past ones?
Look back at your last few boyfriends: do the relationships all follow a similar pattern? Do you always date nice boys but lust after bad ones?
Jodie, 43, would always go out with musicians, actors and models. 'They were all talented and had troubled pasts. I was always trying to motivate them or throwing away old pizza boxes from their living room.'
In effect, they had all made her feel like their mother rather than an equal partner. She fell for their potential rather than the reality.
Her current boyfriend was cut from the same cloth. Instead of trying to work on her boyfriend, she dumped him and worked on self-improvement instead.
She realised that there was a similarity between these bad men and her father, who had been a serial gambler. By understanding the past, she could stop herself from endlessly repeating the pattern.
GO NO FURTHER WITH HIM: THE TELLTALE SIGNS
Don't even start to consider him as a long-term prospect if you spot any of these traits:
• He gives unasked-for reassurance about his behaviour. He tells you 'I'd never cheat' or 'I don't drink as much as so-and-so' even though you've never brought up these subjects. STOP: it's his guilty conscience talking.
• He has a poor opinion of other women. Does he bad-mouth ex-girlfriends? Is he rude to waitresses? STOP: He has a black and white attitude towards women and it is easy to get on his wrong side.
• You find yourself making excuses for him. Do you feel the need to tell friends that he's 'not always like that' or 'he's had a tough upbringing'? Do you downplay bad behaviour? STOP: Your friends are worried about you.
• Something in his life doesn't add up. Does he disappear for a few days for no real reason? Have you repeatedly caught him lying to other people? STOP: No trust, no relationship.
• He always has to win. When you argue, will he stop at nothing - including cruel remarks - to get his own way? Does it seem that he doesn't really take your opinions seriously? STOP: Relationships need give and take.
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• These tests should help cut through the fog that can obscure your view of the future.
Hopefully, your beau has passed with flying colours. If he has failed on a question, tell him how you are feeling.
Maybe you will get a pleasant surprise and he will fight for you; maybe he will agree that the relationship has no future.
Either way, you will have clarity. If you are still undecided, it's probably because in your heart you know the truth but are not ready to face it.
