If we knew the definite answer to this question, there would be no need to keep searching on dating sites. Obviously the answer is different for everyone, and what applies to you might not work
for someone else. But there are a few things that can be looked at, that applies for most people.
One of them are interests. Sharing interests with another person can definitely be an indication that you go well together. This is even more true for values. If you have the same values as your
partner, and appreciate the same (or similar) things, it's more likely that you will get along. But sometimes the opposite is also true, and in those cases the couple manages to complete each
other instead of adding to their beliefs and values.
Sometimes a gut feeling can tell if the other person is someone you would like. This is more of an intuition than an analytical process. But this happens more rarely on dating sites than in real
life, because you seldom get the full picture before meeting someone in person.