Good hashish (or hash) usually consists of pure resin, gathered from a female Cannabis Indica plant (these generally produce more resin than their Sativa counterparts). Commercial hash often, if not always, contains additives (varying from pepper to coffee to rubber), so be careful what you smoke!
In theory, the effects caused by smoking hashish should not be very different from those experienced when smoking cannabis, although they may be more intense. It is, in effect, a concentration of the psychoactive chemicals present in cannabis, so you are just leaving out the plant matter (and some flavor) and increasing the cannabinoid levels in your smoke.
Hashish in a beige, powdery state (bubble hash) is considered to be the least psychoactive. By pressing it together until it turns to a dark brown, almost black color, you break the resin glands and release the oils inside. Hash in its pressed black state is presumably the most potent. It must however be noted that this is the result of oxidation, storing your hash in this state will make it decompose (into CBN) faster. Hash is best stored slightly compressed into a block, while still maintaining the beige color. When rolling it into a joint, one can simply cut off a piece and process it.
Pure, pressed hashish can vary in color from beige to almost black, it should feel harder than a block of rubber, but you should still be able to bend it somewhat when applying ample force. However there is not really a way you can see or feel, or even smell that you are holding pure hash. Even the taste can not always guarantee it.
If you want to know what you are smoking, you should make it yourself! Minimal amounts can be gathered even by processing the small, harsh-to-smoke leaves from buds. There are many ways to make hashish with plant material at your disposal. One could simply rub the resin from the plants, run the plant matter through a series of sieves, or separate the resin glands from the plant by means of more exotic methods such as ice water hash extraction.