Truth is – you don’t “know.” You hope. You use your brain and your instincts, but for most (all?) of us there is no “aha!” moment. You need to ask yourself honestly and repeatedly if you believe that this person is truly right for you, if you can see yourself living with him/her for the rest of your life, and if this is really right for you. There will be no clap of thunder or beam of light that will tell you that this is your bashert. Even after marriage, you are likely to find yourself questioning.
The best piece of advice that I ever heard was that rather than questioning your decision when times are rough (and I mean regular rough patches, obviously not anything drastic), tell yourself that your spouse was meant for you – don’t wonder if s/he is. Wondering will set you on the fast track to dissatisfaction and c”v destruction.
Rabbi Bursztyn, the principal of BYLA, is well-known for saying “if you’re seeing stars, all that means is that it’s dark outside.”
Don’t wait for a magic moment. It doesn’t (normally) happen.