There are over 2,700 species of snakes worldwide. They are found in every country except Antarctica, Greenland, Iceland, Ireland and New Zealand. It's not surprising to find a snake skin here and there. The good news is is that of the 2,700 species of snakes, only 375 of them are venomous. There are several things you can look for that will help you determine what type of snake skin you've found.
Determine the size and shape of the skin. There's a great chart in the links below that will help you narrow down what the snake skin could possibly be. For instance, garter snakes generally fall into the medium (1 to 3 feet) category when it comes to size. There are other indicators such as head shape and the scale texture.
Look for the snake's color and pattern. Snakes can go from simple muted singular colors to vibrant and very identifiable patterns on their scales. The patterns can be on either side of the snake and on the back or the belly.
Find any special identifiers. Snakes vary by the shape of their eyes, their undertail scales, anal plate and their tail end. Looking for these varying characteristics in your snake skin should help you narrow the odds of your skin being that of a particular snake.