Depending on the music, sometimes the tweeters don't play at all or it's not as obvious that the tweeters are playing. However if you do play some treble high music...a good way is to do(at least what I do) is wave my hand back and forth with tweeter, during high frequency music...or...set the EQ right bars right up top, to see if indeed they are working or not working at all.
Oh just re read it...did you put the sound focus to 'driver' or 'all' and is the placement of the sound smack in the middle?
Otherwise, if both tweeters on the same side don't work, I wonder if the lead is disconnected.
Did your tweeters on that side never worked? Or just recently it has stopped working?