I have a reasonable set of speakers on my music pc but not monitor quality. I set them up with a commercial recording i like to get the eq right and then i know i'm comparing my mix with the professional one. But i mix mainly on a good set of headphones and then as floyd says try the mix on the speakers.

I also do a test by making an mp3 of the mix (yes I know p3 loses a lot according to purists but i can't really hear it!)

then i play the mp3 through my big hifi with an ipod. at each stage i might hear something to change and like floyd says the improvement usually show when i return to the headphones.

If i could afford a set of studio monitors and a dedicated studio listening environment I would probably just mix on the monitors. but i would also try it (as I used to when I was the client at professional recording sessions)on small speakers so i could judge what it sounded like on the tv or radio.

but i have a limited budget so i do the best i can. most of my stuff goes onto cds for personal use and i always have the option of remixing if after repeated listening i hear something i don't like.

and after all, i'm an amateur these days so as log as i can get a sound i'm happy with i'm quite content.