tweak: as j swift says good pills, ie those with MDXX in them have a very distinctive bitter taste, a bunk pill with little or no aactive ingredient will have little or no taste, as it will be mainly filler and salts.
but as j swift points out many other chemicals have a bitter taste, the trick is to work out which taste is the one you want...
but remember that this methos is far from foolproof and you generally have to have bought the pill already to be biting it, so ez-test is by far a more reliable method and should always be use by preference.