Most mixed breed rabbits don't have dwarfism. Usually, even if they are a cross of a dwarf breed rabbit, the 'dwarf breed' parent was a false dwarf of its breed, which are more common in pet lines.
But if you have reason to believe that it could have dwarfism, the best indicator, I've found, are the ears. Dwarf breed ears are noticeably shorter and thicker than their siblings would be, or any non-dwarf mutt. Take mini rex: those big, normal looking ears that some have (ears longer than the head and sparsely furred, like for example, in this picture:
https://www.pets4homes.co.uk/images/cla ... d8b04e.JPG ) indicate non-dwarfs, while the shorter mini rex ears (like in this picture:
http://www.johnnyhaussener.com/uploads/ ... 1_orig.jpg) indicate dwarfism. The ears, imo are the most reliable way to tell, but (especially if its crossed with a bulldog head breed, like netherlands or hollands) longer, thicker fur, especially on the head can be a good indicator of dwarfism too. Such that they tend to keep their 'baby fluff' that makes their head look big into adulthood.
Of course, the best indicator that mixed breed is dwarfed is if it throws a baby peanut