How can you tell in advance whether two people will be compatible--or incompatible--if they get married?
You can, and cannot, determine this. On one hand there's common sense and research, and on the other there's the infinite plasticity of the human mind. This is to say that we can have a pretty good idea of compatibility, but that this will not necessarily hold up over time or changing conditions; otherwise we'd have a nice flowchart of marriage compatibility with which everyone could save themselves a lot of time and alimony.
Naturally this is a point of research because of the changes in lifespan and the nature of domestic life in recent times (say the last 100 years or so). There is generally a perception that divorce rates are high, or at least higher than they used to be: some statistics place divorce rates as high as 50% for all marriages, and this is not even to say that the 50% who remain married are necessarily happy. I've included a link to some statistics on divorce rates, but there's an enormous amount of research into this subject should you care to investigate it. The only problem is correlation and causation.
It can be especially difficult with humans, partly due to our aforementioned plasticity, to determine if a correlation is a causation. For example, we could conduct two different studies, one asking "what traits lead to happy relationships?" and the other asking "what traits lead to happy marriages?" You'll probably get different results because of the different priorities people attach to their own internalized definition of a marriage. You'll also get very different results based on culture and probably location as well. Likewise, the things that lead to a divorce are not necessarily the opposite of the things that lead to a happy marriage.
Thus, I would argue that one main point of determining happiness (which might seem a bit obvious) is if the two people agree on the terms of marriage itself. Does marriage mean the man is in control of the house, or that the woman will change her last name? As long as we're determining the success of this relationship based on the success of the marriage, then both parties need to be agreeing to the same marriage.
We could also point to factors like age at time of marriage, or financial stability, which affect marriage happiness. However, these are common because they're easy variables to research, not because they necessarily tell us about the people themselves; we might just as well observe that redheads have happier marriages than brunettes, but this doesn't tell us anything about why that happiness is effected. I think the second most important factor is a self-awareness of the terms of happiness; you must know what makes you happy and why.
Third, I think, is the capacity for negotiation. As the third link below states, it's more about dealing with incompatibility than compatibility that determines marriage happiness. Couples need to be able to think in the long term, identify things they are flexible about, and readily visualize how they and their circumstances might change in the future.
An important takeaway from this is that the specific details, like age, religion and so on, often have less to do with happiness and compatibility than the ability of the individual to socially interact in healthy ways. Thus, the best way of determining whether two people will be compatible in marriage is to give them a conflict.
I feel pretty sure that it isn't possible to know for certain whether two people would be compatible if they got married--but there ought to be some way of estimating percentages of possibilities. I think it's very common for all of us to guess whether any two people who are getting married will stay married. We even think about this question when we are attending a wedding ceremony. Naturally everybody says, "I hope you'll be very happy," or even, "I know you'll be very happy"--but do we know? Can we even guess? And, if so, on what basis? And some couples will stay married even if they are not happy together. They do it because of the children, or because of religion, or because of parental expectations, or social pressure, or something else. I don't think that people who haven't been married can really understand what marriage is like. I have been married. Once!
There isn't a way to determine with 100% accuracy. There's not rhyme or reason for compatibility either. That's why "Birds of a feather flock together" is same amount of true as "opposites attract." Marriage is more than a simple compatibility issue in my opinion. It's a commitment to stay together and work through things even though the two people are not completely compatible in their personalities or viewpoints on particular issues.
There isn't a true way to tell whether people are compatible or not. The more you are around a person, the more you learn about them and your feelings may or may not change. You may like a person initially, but the more you learn about them could change your views. It also depends on their actions in the future. Often people change over time and you never really know the decisions they will make and you'll never know everything about them. You can never really be sure how you will get along in the future.
Everyone is different with different views, habits, thinking. You can not predict compatibility. Marriage is a great thing but one should not care about compatibility, I mean that when you love a person and marry him. You will find new things and habits in him nearly everyday. But if you are in love then you will ignore this compatibility thing and will change yourself according to his choice. You will automatically adjust to his thoughts. When married both will have to cooperate with each other and that only will make your relationship strong. By cooperation I mean both pupil like sometimes man sometimes woman both should adjust their choices by keeping their life partner in mind.
Some people study the stars that both pupil belong to. and their are software's to measure compatibility. You can do so but human nature is like very deep, it keeps on changing with different situations. Once compatible life partners thoughts can change and they can have issues. Therefore the main points in Marriage that is supposed to be kept in mind are compromise, trust, love, respect, etc for your spouse to keep your relationahip healthy and blossoming.