Clawed frogs also come in brown so you can't go by the colour. I just got my dwarf who was less than an inch long , he was in with clawed frogs which were around an inch and half. Dwarfs have webbing between their hands, clawed don't have webbing and their hands look more like fingers, also the eyes are positioned different on their head. Take a look at the pinned topic at the top of the page for pics. A dwarf frog could live out its whole life in a 1 gallon tank. Clawed frogs get a lot bigger and would probably need a 20 gallon once fully grown.
Hope this helps,
I know how difficult it is trying to find dwarfs as the people in the shops don't even know what they're selling. My dwarf came on their list as brown frog which wasn't much help, i have googled 'african dwarf frog' loads of times but still have a few unanswered questions about them.
Emma