How to know if IPhone is blacklisted?

I bought an iPhone 4 from a guy but when I went to unlock it the guy at the unlocking place said it was blacklisted how do I know it's really blacklisted its and IPhone 4 for AT&T?

You would know if no active SIM card works in the iPhone (it says No Service no matter what sim you put in). There are also services that you can pay for that will tell you whether or not it is officially blacklisted (reported lost/stolen)