I saw this post on HowardForums a little bit ago and figured it would be a good idea to share it here. The guy who made it seems to a reputable member over at HoFo, so I'm sure there is no danger in using it to quickly check if a Sprint ESN is clean or not. From what I've read, this only works for Sprint, Boost Mobile, and Virgin Mobile devices.
Here is the link to the website:
Online ESN/MEID Checker by deathtrip
Here is the link to the thread over at HowardForums:
Online Sprint, Boost, Virgin ESN/MEID Checker by deathtrip
Even when using this tool, I would still recommend verifying the status of an ESN by calling Sprint before purchasing a phone. It never hurts to double-check...