It's a risk. There is no surefire way to know if a guy will be that serious. There are some signs though, co-signing anything, a ring obviously, or letting you meet his family or meeting yours. These are signs that he's taking the steps forward.
I don't know what kind of signs I give off but I can tell you what I do. I am going to marry my girlfriend next year and I treat her the way she deserves and she knows that she is the only one for me. I trust her and she trusts me which is one of the most attractive things about her. I take her any where she wants buy her anything and we spend a lot of time together so we really know everything about each other which helps if you want to take the relationship to the next step. I just bought her a brand new car we have a wonderful house and have been living together for about two years now. I never forget our adversity or her birthday and I always do something special for her. I give her flowers regularly we go out to diner once a week and do other things. I tell her that she is the most beautiful woman and that I am the luckiest man on earth and I really am. The biggest thing I guess is that I involve her in all of my decisions so she knows I am going to be with her forever. Those are the things I do to show my girl that love her and that I want to spend the rest of my life with her.
It's all kind of cheesy now that I actually read it, But damn I lover her.
Its a gamble your age does not matter how long you have been together I've known people that have been together foe3 years get married and divorce in 6mos It's a gamble you hope he/she is into you as much as you are into them