The market is hot for signed pieces with names like Miriam Haskell, Coro, Trifari,and my personal favorite, Eisenberg. Bear in mind however that all pieces are not signed.
In that case look for quality in the manufacture and be wary of excess scratching.
This bracelet has been in my collection a long time and I love wearing it despite the lack of a signature, it is very well made.
Most good quality pieces these days will sell for far more than they did in their heyday.
To clean your costume jewelry, use a small dab of toothpaste combined with one drop dish soap and swish in some cold water, rinse in cold water and blow dry on the air only setting. Heat will damage them. As will perspiration. Make sure they are completely dry and store in a safe place until you are ready for a night on the town!