The only way to know for sure if a piece is real trinitite is to check it with nuclear energy spectroscopy to measure the level of radioactive decay. From what I understand, trinitite produces a very specific pattern that cannot be faked. (I'm not a nuclear physicist so I don't understand it exactly but people have created fake trinitite in the past and made it slightly radioactive so this is the only way to be sure.)
The reason I myself found this interesting is because the description of this material almost exactly matches a rock I found on a short fishing trip with my father about 16 years ago. Only problem is though, I found the rock in a stream in southeast PA. ;) Spooky huh? hehe
You can apparently buy some trinitite, although, even though the same website warns against fakes, whos to say they're not selling fakes. hehe Again though, the pics they have there look almost exactly like that strange rock I found as a kid. (cue the Twilight Zone theme! )