Wasn't really sure where to post this, and didn't see it any searches but I found this while poking around after picking up a phone on Craigslist.
CheckESNFree.com : The only instant, free, online ESN Checker
I tested it out with 2 old phones I have and they came up clean, the one I picked up today said call customer service!
Luckily the guy I got it from is cool and giving me my money back.
But hopefully this will come in handy for some.