There's only 32,768 combinations, so trial-and-error shouldn't take long. Of course, if you had a cluster of them then you could brute-force crack it in much less time...
The dip settings vary between manufacturers and even mac models. Try looking it up on the web, to see if any diagrams are there. I always taped over the dipswitches to keep morons away. An even better solution is to use a Panasonic monitor cable - they have a Mac VGA on one end and a standard VGA on the other. Total no-brainer.