I found this great key signature finder at about.com.
This is a really useful tool for not just piano players but for all musicians. It shows you exactly how many sharps or flats there are for each different key signature whether it be major or minor.
You search for your key signature by name… when you click on the key then you get confronted with a different page showing you exactly how many sharps or flats that particular key has. You also get shown which is the relative minor (or relative major) along with a list of the most common chords for that key signature.
As well as showing you how many sharps or flats it will also show you which they are listing them on the musical stave as well as in writing.
You will find this resource extremely useful if you are starting to read music and are now playing in several different keys. It will quickly serve as a reference to show you exactly how you should be playing each song.