Hi Jock hi all,
advice taken from http://www.ukcia.org/activism/soapbar.htm not all soap is as bad as all that though, so be choosey when buying it.
Don't buy rock-hard stuff, or samples that give off a dark acrid smoke when burned.
Cutting through a block of resin should expose a brown coloured surface (including if you have black hash) and it should fluff up when heated and be easy to powder with just a little flaming.
The smell of pure hash smoke is also very distinctive in its own right, learn what to sniff for!
also if youi smoke it neat in a pipe you should be left with a clean looking white ash in the bottom of the bowl
fester x