First, you need to eliminate some variables, not counting the variable of your hearing, which might need to be checked.
As has been mentioned, play a known monaural signal and see if it is centered. It should be. If not:
If you know-that-you-know that you have the speakers at EXACTLY the same distance from your ear to the tweeter on each side (the tweeter is the primary source of directionality), then these steps should help:
The easist thing to do would be to do some cable swapping, channel-to-channel, until you've made sure that your source, your pre, your amp, etc. have all been swapped left for right (and of course, returned to their former position). Do this one component at a time and listen to the same recording each time. If th eproblem has changed channels. then that component might have an uneven output (or sloppy volume control).
If you still have a problem, and again you absolutely know that you're in the exact center of the speakers, and that they are the same distance from each ear, then you'll need to swap the speakers, being doubly careful to measure distances. An inexpensive laser measuring tool is really useful for this.
By now you've swapped everything, and something should should have shown up.
If it still favors one side, I'd check to see if the slightly louder side has less reflective surfaces on the side wall.
