One of the easiest ways to figure out what snake you've seen is to go to www.google.com and in the search bar field type in a physical description of the snake you saw along with the state you saw it in.

Example: snake red and black checkered state_name and then use the image search feature and see what it comes up with.

This won't always work, but it is a good starting place. If you have a picture of the snake you can post it on a variety of online communities and forums where people will be more than happy to help you out. You could also take the photo to a local expert and ask them what it is or you could just tell them what you know and see if they can help you figure it out.