Spotting fissures; there’s one specific asteroid type - at least in the ice fields - that contains fissures. All other types do not. So learn its shape, and ignore everything else no matter how bright it is. Then if you see one, fly close and look for fissures; you should know what to look for, and with practice you can usually check out an asteroid in a matter of seconds. Only when you see fissures should you launch a limpet. I never waste limpets on prospecting anymore.
As for demand, the simple explanation (well, I think!) is that 3 bars is good for buying (there’s a lot of it so it’s cheap) and selling (nobody is selling it so the prices are higher), and 1 bar is not. I read a more detailed explanation somewhere but that was my take-home. Can’t remember what the figures mean though. :) I’m sure someone else can clarify this.