Its funny, Molly is supposed to be slang for "molecule" which in turn means pure MDMA. However in the South/Central US area it's just become slang for gel caps filled with powder.
The most common slang in my area:
Tabs-Pressed pills
Molly-Gel Capsules
If you've tried real MDMA and RC's(research chemicals) you can taste real quick the difference. Pure MDMA comes from Saffron trees and has a slightly sweet/root beer taste. Methylone or Mephedrone are the common substitutes(aside from Piperazines) and they taste like shit, like they came straight from a Chinese laboratory(and they probably did).
Also another thing to note is that if you get a gel cap that's packed full and it's not tiny as hell, it's cut with something. A size 4 gel capsule will hold around 150mg of mdma which is equal to 2 good "tabs" these days. A size 0 gel capsule which is the most common, will hold ~500-600mg. If you see one of these larger pills completely full, you're either going to pay $60 and you better split it with at least 2 other people or in 99% of the cases, it's filled with something else.
Be smart, and be safe, never trust anyone. Even friends can be deceived by others even if they don't mean to. Plus, if you get a pill full of piperazines you better hold on for one of the shittiest rides of your life.