When you meet the right person, after a while you will "just know."
There isn't a significant event that tells everyone to get married. It's not like the marriage fairy visits and tells us. It's something that one just knows.
A big problem I see a lot is that people mistake the early "love" in a relationship for "just knowing." Give it time and if things don't change and you're still head over heals well after a year or two, You're ready.