People are sometimes awfully hard to read, let's just face facts. But sometimes you don't need to hear them say something to know it's true (or, at least, to think that it might be). Men can send some big-time signals through the things that they do, and there are some weird things men do when they realize they met the one that, if you're in a relationship with one, you may be interested in knowing more about. Because people can be so difficult to read (and because expressing emotions can also sometimes be difficult), even if you think you know quite well that your partner thinks you're the one and sees your relationship lasting long-term, there's still a chance that you'll question things from time to time and find yourself looking for some fairly subtle (or super obvious) clues that this really is how they feel.
When people feel as though they're with the person that's the right one for them, they tend to do things a little bit differently than they may have done in past relationships, with people that may not have been quite as well-suited to them. That doesn't mean that they're acting like a completely different person or that they're acting differently than the person they truly are, but knowing what sorts of things men might do when they realize they've met the one can give you insight into your relationship that you may or may not have otherwise.
If you see the man in your life doing some of these things, it could be a sign that they know that the person they're in a relationship with is the perfect match for them.