But I guess they don't. I have only seen a few snakes, but I had no idea what kind they were
lol. Here's what you do, get real close and look at the shape of the eyes. Round and its not poisonous. Elongated and it is.
Joking of course. It's correct but I always laugh when they tell you that. googled some images for you.
Copperhead (poisonous)
http://www.uvm.edu/~jbartlet/nr260/animal%20life/reptiles/copperhead.jpg
Rattlesnake (black and yellow phase, poisonous)
http://www.masnakes.org/snakes/timber_rattle/thumbs/fullsize/timber_rattlesnake_500_TomPalmer_fs_fs.jpg
http://www.sierrapotomac.org/W_Needham/Pictures/Timber_Rattle_Snake_09_13_03.jpg
There's a handful of Massauga rattlers too, but they're in such a tiny area and so rare they're not worth mentioning. Both types are very fat snakes, and thats the first impression, very fat. They also have larger scales than our other snakes. And a very large, triangular head is a distinctive feature, but it can be misleading as on close inspection other snakes heads can be triangular too, just not to such a degree.
We have lots of types nonvenomous snakes, and most of those types are much more common than the poisonous ones. Water snakes and black rat snakes are the two I think are most commonly mistaken for poisonous snakes. Generally, if its in the water, it's non-poisonous, with very few exceptions. On land it can be tougher to tell, especially since many individuals look different depending on where they are in the shedding cycle.
Water snakes. Typically small head. Skinnier than the poisonous snakes. Always near water. Very aggressive and they like to shake their tails in leaves and such to mimic rattlers. Coloration varies a lot, from light tan, to reddish, many shades of brown, to almost black. But they usually have a pretty strong pattern on them.
http://www.dpughphoto.com/images/northern%20water%20snake%20durham%2070607.JPG
http://farm1.static.flickr.com/51/156938918_b27869f228.jpg
Black rat snakes. Typically solid black with a white chin, but they can have some patterning when they just shed or they are wet. These guys can get big.
http://www.michigan.gov/images/black_rat_snake_101499_7.jpg