You asked a very good question that many of us ponder whether we're in the garden turning the soil, walking outside after a rain storm, or just playing in the backyard turning rocks over (I still do it). Well, actually that's not an easy question, seeing how just about all worms look alike.
I would first start by looking at the size of the worm. You asked in particularly about how to tell if it's a red wiggler. Red wigglers don't get very big at all, usually around 4 inches and very spindly.
But what if it's a baby nightcrawler. Could that look like a red wiggler?
Yes, it could to the untrained eye perhaps. Look at the clitellum (the thick swollen band or ring around the worm) some call it a "saddle". Does it have one? If so, then you could possibly have a red wiggler (Eisenia Fetida). If it is big and as fat as a finger it is probably a nightcrawler.
Red wigglers are surface dwellers and only stay at the top of the soil where rich decaying matter is found and if you have hard winters or hot summers these little guys probably won't make it through to the next year except for their cocoons. Some earthworms live in the soil and build lateral burrows while others build deep vertical burrows that can extend up to 6 ft.
so, I hope that may give you a little criteria to go by. If not, then their is a cool little chart with pictures from Opal Explore Nature.org you can follow by using the process a elimination.