I had the same question several weeks ago, this is what their techs quoted;
"The best way to tell if it is our upper receiver is to look at the inside of the upper. The inside of the receiver should be a slightly lighter gray than the outside. This is because we bake on a Mil-Spec dry film lube to the inside. Other manufacturers do not go with the extra cost to do this."
The receiver I bought didn't have this coating and confirmed my suspicions The height from the bottom of the receiver to the top of the rail was off .040 from a brand new factory Busmaster upper I later received as well.
After contacting the dealer I bought the so called Bushmaster upper from and having him still tell me it was a Bushmaster, I told him what Bushmaster said, he then quickly told I could return it because he "let it go too cheap anyways" and ended the conversation. When I returned it he didn't believe me when I told him what I had paid for it, but luckily I still got all my money back without a hassle. This was at a gunshow and when I wanted to show him my email from Bushmaster, he just turned away, it seems to me like he didn't want too many of the potential buyers around his table to here about this. Maybe he was ripped off himself or maybe he really knew, since he only deals in AR15's I think he probably knew it wasn't a Bushmaster but I can't prove that. It goes to show though that when buying an upper make sure you have a reputable dealer and find out what to look out for when identifying a bushmaster upper. Just because the barrel says Bushmaster doesn't mean the reciever is as was in my case. I think all this could be a lot easier if Bushmaster would just mark their uppers in some way.