I would know in a sec if she was cheating on me, I would feel it the 2nd I saw her.
Ok, time for some 'tough love' here...
If you weren't even able to 'feel' that she wasn't into marrying you -- how could you possibly know if she was cheating or not?
If you have been dating a girl for 9 yrs and she hasn't even hinted around or given any clues that she would like you to propose then she never wanted to marry you.