If the PA is reasonable....you should be able to run your mix (or a commercial CD) through it with the channel EQ's flat...and it should sound good..if not, something , somewhere needs attention..follow Macs advice and run a commercial song through your PA that you like and are familiar with...

Your stereo mix, should sound good in mono....if not, you need to addresss your pan placement, stereo effects, or you have phase issues...its a good idea to always chekc your mixes in mono...

but, a good monitoring system..in a well thought out and acoustically treated room (read...it doesnt have to be expensive), goes a long way to getting, good , translateable mixes...

cheers

Wiz