Although I don't formally believe in monogamy, (despite I sometimes delude myself into practicing it)--I also don't believe in lying to someone, and telling them they're "the only one," when they're not. And with this situation, why bother with "evidence from her past" when you have evidence from the PRESENT, nay, even from YOUR life? I consider a person's past "stats" to be absolutely pointless and irrelevant (BECAUSE they can lie about whatever they want, just for one, therefore I never bother quizzing someone about their past relationships, or playing Detective Snoopington with their personal affairs--and I EXPECT the same from them)--AND this situation is no different.
Why? Because you have your OWN experience with her--you ran into her friggin boyfriend for pete's sakes! Not her ex, her CURRENT one while you were also her CURRENT other one!!! (If I am understanding you correctly!!!!) Therefore, why do you even need to ask whether or not she is lying to you--even if she isn't CURRENTLY doing so, she HAS ALREADY, and you saw it for yourself!!! I know romance is not a rational situation, but that doesn't give license to chuck reason completely out the window!!!! Hell, you don't even need to check her cell phone or some bullshit (not that I would recommend that anyway, as, FOR ONE, a call can be deleted out of the "history" just as easily as a little text, so that is simply another fool's method--not to mention ridiculous behavior.) Even background checks for things like employment can be gotten around with a simple identity "fudge," so I always say YOUR BEST AND ONLY REFERENCE IS YOU. And you have personal experience with her doing this already--silly "detective work" be damned, what more do you need????
(I suppose you could make a fake online profile pretending to be a random guy, and hit on her (from a public computer of course!) and see whut she says. BUT if you have even gotten to the point of considering that, the relationship sucks, therefore there is no point in actually doing it. If that is even an issue, you know?? Which as far as I can see, it was certainly an issue when the "relationship" began--I have no idea why you have even allowed it to get THIS far, you know??)
Also, keep in mind that her saying she loves you (even if she thinks she really does and is not "lying" per se) does not equal considering you her boyfriend. Has she referred to you as such, either time? What "future plans" do you mean--getting engaged, seeing you next New Years--what? You don't need to tell me, just be specific with yourself. A year ahead is not "future plans"--at least not as they are usually seen with respect to a romance. It's NEXT YEAR. Soooo believe it or not, you may not actually be "in a relationship" with her--BUT don't get into one, should that come up. Unless ya wanna BE the one bumping into her and some guy at the fair...??? Speaking of fairs, not to be a buzzkill, but from here I'm smelling what we like to call a "summer romance," which can be a beautiful, fun thing...as long as it is seen for what it is.
I could be wrong, but...hell, you ran into her friggin bf at the fair! What more do you need. Seriously.
The good news is, your trusting her after the first experience of directly encountering the "other man"--is evidence to me that you have either conquered your paranoia or are well on your way to doing so. Keep THAT part up. :) In that respect, you have gotten MUCH farther than many others who struggle with paranoia for years.