With the exception of coral snakes, poisionous snakes usually have triangular shaped heads. It's a quick easy way to tell.
I found a skin that some snake shed right near my front door. It must have been about 5ft long. I tried to pull it out to see if there was a rattle on the end but it broke up. I was talking to one neighbor at work who saw it. He doubts that it was a rattlesnake because when they get that long, they are usually fat, this skin probably came from a rat snake. (I hope) I have found several baby rattlers a few years aog, let's just hope that this isn't momma.