How does one go about knowing whether they're poly or not when they've only been in two monogamous relationships?
Part of the answer is: you can't "know" for sure; you just gotta try it out and see if it works for you! I'm guessing you didn't agonize about knowing whether you were monogamous or not before getting into monogamous relationships - the point of dating is to try things out and find out what works for you!
The other part is: the same way you find out anything about yourself - introspection, daydreaming, reading, testing, experimenting, exploring. Check out my FAQ about this here.