Good luck, I posted a question like this once, but the answer isn't easy.
What I can tell you is this: You'll know when you know.
I know that sounds sarcastic, but that's not my intent. It's hard to explain an easy way to tell, without taking some home and testing it, but once you've seen the difference with your own eyes, it's pretty obvious. I've personally only seen Silnylon at Jo-Anns or any other fabric store once. PU coated Polyester I've seen as fabric for making raincoats. All the nylon they carry is generally either un-coated, or, DWR treated. The stuff that feels slicker on one side, but you can still blow breath through easily, is probably DWR.
The PU coatings I've seen, look noticably coated, like plasticky on one side. DWR doesn't change the fabric look, but the feel is different, same with Silicone impregnated, but the feel is even more drastic.
Dunno if that helps, might want to try to eyeball some PU coated polyester (which I've seen lots of at Jo-Ann), if you have to be waterproof, or use something else. Lots of other options for a tarp besides nylon.
Hope I got all that info right, good luck!