Here are the sloughed skins of a female adder and a grass snake. I can tell the difference between these snakes within a millisecond, but for all the people that are unsure, these pointers will help you id a snake if you are lucky enough to see one in the wild.
Dark green / olive colour with dark dotted scales running down the body.
Yellow collar around the back of the head. Vibrant in younger snakes, duller in older ones.
Orange eye with a round pupil.
No V on back of head, like the adder.
Females are brown, with a darker brown zig-zag pattern running the entire length of the body. Whereas the male adder is grey with a dark, almost black zig-zag.
Red eye with a vertical pupil.
Has a V on back of the head.
I will post a photograph soon of the adder and grass snake together.