Jeepman:

Look at the note of your bass string. The notes from the nut toward the body of the guitar are E, F, F# or Gb, G, G# or Ab, A, A# or Bb, B, C, C# or Db, D, D# or Eb, E. (I've left out double #s and double bs. If you are playing in the E position, whatever note your bass string is, is also the chord you are playing and the key you're playing in.