1. find out porn name of the girl
2. google " free pic"
3. prefer results from peachyforums or freeones
4. open up some pic gallery
5. examine the face to your heart contents
6. be 99% sure (that 1% doubt will always be there since you're emotionally involved in the assesment and you hope you're wrong)
7. proceed depending on the results of assesment
8. i'd recommend more indirect approach, sending her a link and saying "hey, this girl looks like you, it's incredible", but she can shrug it off with "yeah, she does" even if it IS her, so it relies on her being sincere and confessing... well, any way you can ask relies on her being sincere, but this one gives her more room to not tell the truth if it is her.
alternative:
1. find out her porn name
2. google the agency the girl's with
3. contact the agency, pretend you want to do a porn/erotic shoot/movie
4. ask for contact info
5. then you should be pretty much sure
but this one i don't know, never tried it, not sure how the agencies filter out people who really want to do some work with the girls from the fans, and you may not succeed in the 4th step.
another alternative:
say it like "i saw this girl in porn the other day, and she looked like you, too much for me to ignore it, and... i know it's a stupid question, and answer is kind of obvious, but it bugs me and i need to hear it from you... i wouldn't even mind if it was you, but it's the uncertainity that's killing me, i feel paranoid and can't get it out of my head".
basically - i assume it's not you, but you know how human psyche sometimes works, it won't let me rest till you tell me.