Strange question. Can one see if a DVD consists of 2 layers without using any electronics but your own eyes? Is it possible to turn DVD with data side up and see if it is one or two layered DVD?
IIRC one-layer DVDs are a silver color and dual-layer DVDs are gold. Not sure if that's correct.
Normally, there is a bar code or label below the discs.
Double layer DVDs have two bar code/labels below the DVD ( the reading side ).
I think this is the way to identify.