Either (i) find someone with perfect pitch and ask them, (ii) look on the web for a guitar tab or similar, or (iii) play along with it and unless you're tone deaf you should be able to tell which key sounds right.
the song is one of my own.... ive recorded it and the engeiner was going to use an autotune of the vocals and asked what key the song is written in and i didnt know what to tell him. the chord progression of the song is ::: C#, A and thanks for the quick reply