2. You make long-term plans together
The realization that you're ready for marriage may actually be obvious. If you've initiated or willingly participated in discussions with your girlfriend about eventually buying a house, having children, saving for retirement or any such future plans, then it's a logical assumption for you (and a fait accompli for her) that you should propose. Similarly, if you've found yourself paying special attention to reports about the suburbs (hikes in taxes, school zones, housing markets, etc.), then don't fight it any longer; you're ready.
1. You know she's the one
While all the prior examples are good indicators of readiness for marriage, nothing is as important as this one. In essence, this reason is really the assemblage of all the prior indicators; the reason that all signs point to her. After all, you're with her now for a reason. She is the culmination of all the dating experiences that came before. You've learned who you are; you remember the girls you've thrown back while waiting for that special catch. And now you know that the "one" is in your life. All her good traits and the way she makes you feel about yourself impress upon you the absolute need to keep this one. There was never anyone like her in your life before and you know there won't be again. So stop dillydallying and get to it, man.