[musik_maniak]

Yes, as Five says, you can listen to it on headphones, and if you have a "mono" switch on your stereo amplifier, you can switch that "on" and "off" as you listen, and see if you hear a difference. If nothing changes as you switch back and forth, then it's mono (and it's good mono at that, because if it's unbalanced mono, you'll notice the sound moving from left to right, but it still won't be stereo).
So if you hear a change as you switch from "stereo" to "mono", listen to the individual instruments in the mix, and if the different instruments move to different places in the stereo spectrum (left and right) then it's really stereo, but if the whole mix just moves left or right then it's likely unbalanced mono (which really sucks, by the way).