The most dependable way in this area is to look at the anal plate scale. We'll talk assuming it's native and not an exotic released. If it has 2 rows of scales behind(toward the tail) the anal plate, then it is non venomous. If there are 1 row of scales, then it is venomous. Check out this link.
http://fw.ky.gov/pdf/kysnakebook.pdf about a third of the way down. That is assuming it's native to this area and this link is about Ky, but should be the same I beleive. Now, if it were somthing along the line of a type of boa or python that is not native, it will have only one row, but of course non venomous. I have noticed that with rattlers, they have more stretch/skin between the scales of loose sheds, than say a colubrid like you would find in this area. Also, when you find them in the wild, the skins always apear like they belong to a much larger animal than they actually do. If you have a piece of the head, you may be able to see the shape of the head and the pit sensor in the shed skin. It's hard to tell what species they are, but if I were to guess of the top of my head, I would say black rat. I don't know your area, but if I see a snake in my area(N. Ky) it's almost always a black rat or a copperhead.
Jacob