How do you tell if a turtle is a snaping turtle?

Here's a description of a snapping turtle:

large head, long thick neck and back of shell has sawtooth edge. The height of the head end is a lot bigger than the tail end.

There is a good picture on the "Nova Scotia Turtles" page of the Nova Scotia Museum of Natural History website.

Scroll to the bottom of the Museum website and click Nova Scotia Nature; scroll to the bottom and click on Nova Scotia Turtles, Click on "fresh water turtles" and scroll down to Snapping turtle.