How can I find out if my "new phone" has been used before?

I recently found out that two of my friends who bought “new phones” from MetroPCS were scammed because the phones were actually used and one of them had two numbers previously registered to it. My purse was stolen today with my cell phone in it. I had to buy an emergency replacement phone from BoostMobile but as I opened the youtube app it froze. I figured that if its a new phone it shouldn’t have frozen. So, my question is, if I have the serial number for this phone, where online can I find out if this phone has been used before?