Do you understand key signatures, i.e. # one sharp would be G, or Em.
## two sharps would be D, or Bm.
If you are playing from sheet music more than likely you will have a key signature at the beginning of the clef. for example"
http://www.freehandmusic.com/Product...?ProdID=380651
This lead sheet has bb two flats which lets you know the song is in Bb or Gm.
Or you take the chords (or notes) and write them on a piece of paper, cross out all dumplications, omit all the fancy stuff (Bbmaj7#ll all you really need to worry with is the Bb) then put them in alphebitical order. Once you've done that go to your handy dandy chord (or scale) chart and find which one all the chords or notes fit into. Not some - all. That is your key or scale.
Your Cm and Gm7 would indicate a good chance of fitting into the key of Cm.
Cm, Ddim, Ebm, Fm, Gm, Ab, Bb, Cm