The purpose of the detector is to output a voltage that is proportional to the FM modulation, i.e how far the carrier is from the nominal frequency. It can go a certain value up and a certain value down, but outside these limits the signal starts to distort as the detector goes out of its linear operating range (the curve looks a bit like an S). This adjustment is there to set the operating point in the middle of the linear range, otherwise a strong modulation will clip at one end or the other, creating distortion. The effect can be similar to if you just tuned a little bit out.
If you have an FM signal generator - it doesn't need to supply a stereo signal for this adjustment - you could make it supply a strongly modulated signal, look at the detector output with an oscilloscope and adjust so that it doesn't clip at either end of the demodulated audio signal swing (or clips equally much at both ends).
Lacking these instruments, you'll still get a pretty good adjustment just by ear. Mark the original position of the trimpot if you haven't touched it yet. Tune to best possible signal, adjust the pot for best audio quality, maybe repeat a few times
Here's a pretty good writeup on FM detectors.