Irishfield described a good/simple test on this site somewhere. I think it was something like....put a bit of water in a mason jar, mark the level of the water, fill the rest of the jar with gas, shake it up. If the water/gas separation is higher than the original line, you have ethanol in the gas (which mixed with the water, increasing it's volume). If not, you have pure gas.
Or something like that. Maybe he can chime in.