Sex life, religion, money or just taste in movies -- there's any number of ways to predict if two people are compatible or combustible.
For couples therapist David Kavanagh, however, just one session on the couch before they say 'I Do' can determine whether they'll go the distance -- or wind up in the divorce courts.
"I've had couples looking for a refund before even taking our pre-marriage course," says systemic family therapist David of Avalon Relationship Consultants. "The very fact that they had to go to the effort of sitting down to discuss their issues made them decide to split up."
Here in Ireland, talking before you walk down the aisle is compulsory if you want to tie the knot in a Catholic Church -- with the couple geared up for the realities of marriage either by the local priest or at a pre-marriage course.
But further afield, where no such box-ticking exists, couples are still signing up to defuse potential flashpoints before swapping vows.
Hollywood star Jennifer Aniston and boyfriend Justin Theroux are the latest twosome to reportedly volunteer for pre-marital couples' therapy -- despite the fact that they've only been dating a few months.
Given that even her late dog Norman saw a canine shrink at $250 a session, Aniston (42) is perhaps an unsurprising convert to the trend.
But while there are no official statistics, anecdotally, such pre-emptive therapy appears to work.
Stateside, Marriage Savers reports that divorce rates have plummeted by up to 37% in areas where religious leaders have signed a 'Community Marriage Policy' making the sessions mandatory for couples wishing to wed in a house of worship.
So could the Catholic Church be ahead of the curve for once?
With the average couple spending 250 hours planning their big day, what's a few more to help divorce-proof your marriage, say experts.
"Last year, we delivered 16,317 hours of marriage preparation to 13,963 couples in Ireland," says Anne Coleman of ACCORD Catholic Marriage Care Service.
"Pre-marriage courses educate couples in the skills, habits and attitudes that lead to happy, lasting marriages."
"It gives couples the chance to look at how they communicate with each other, explore their understanding of the commitment they're making, discuss their spiritual beliefs, learn how to manage conflict and be responsible parents.
"Even couples who are not getting married in the church often go on our marriage preparation course, which runs over nine hours and costs €80-€200. Marriage research has shown that a good pre-marriage course can reduce the risk of divorce by up to 30%," she adds.
"On our feedback forms, participants often say that they thought they had discussed every issue possible with each other, but discovered they had a lot more to learn about their future husband or wife."
"Think of it as like servicing your car," explains relationship expert David Kavanagh, who hosts country-wide group and private pre-marriage courses costing €120. There may be nothing wrong with it, but it helps keep things ticking over smoothly. I've had couples experience a 'Eureka' moment during the course, where they realised they were making a big mistake -- and I think a good pre-marriage course should allow couples to make that decision before it's too late. People from broken homes want to avoid repeating the same mistake as their parents."
As one such newly-wed, Prince William (29) reportedly sought to weatherclad his marriage to Kate Middleton with regular counselling from the Archbishop of Canterbury and Bishop of London in the run-up to their nuptials earlier this year.
As well as pre-marriage courses which prepare couples for the rocky road ahead, both ACCORD and Avalon offer pre-marriage counselling to deal with more specific problems.
And though it may be about as romantic as unblocking a sink, divorce lawyer John Lynch of DivorceinIreland.com reckons pre-marriage counselling should be mandatory for all lovebirds planning to sign on the dotted line.
"From my experience over the past 30 years, I firmly believe that if more attention were paid to marriage counselling, both pre and post, the destruction caused by divorce would be less evident in our society," he says.
"Marriage counselling before and during a breakup should also be mandatory to offer people a better chance of an amicable outcome."
UK-based divorce lawyer Vanessa Lloyd Platt goes even further, arguing that school children should get Marriage Education -- not just Sex Ed.
'We live in an age where we think everything should be perfect, especially relationships," she says. "They ought to have pre-marital training as early as school to realise that marriage isn't all romance and flowers, that there will be tests, but that such external conflicts can strengthen what you have."
"Pre-marriage counselling isn't for everybody," adds David Kavanagh of Avalon Relationship Consultants.
"And it's famously difficult to get Irish people to open up about their problems. Given the emotional and financial devastation of divorce however, the alternative doesn't even bear thinking about."