There is no good way to find this precisely, other than perhaps using the mirror technique I posted about a year and a half ago. However, there is a way to find a close approximation to the correct focal point.
If you make a few assumptions, and somehow know what the focal length of the dish is, perhaps from the manufacturer's specs, then, I put a calculator at:
BJ Offset Angle and LNBF Position Calculator
This calculator assumes a couple things, but it can tell you if the arm is locating the lnbf at the wrong place.
However, if you don't know the focal length, which I suspect is the situation here, you may have a problem, since the above calculator requires you to know the FL. But if you assume that you have the proper lnbf arm, make the measurements described, and the calculator will give you a distance that the lnbf should be from the top edge of the dish. If this is off significantly, then it would suggest that perhaps you do have the wrong lnbf arm, and if it is pretty close, then you are probably OK.
This is something I just put together in a few minutes, so I'm not positive it works all the time, but it did seem to check out with my dish.
If all else fails, you might wait for a sunny day, and try the mirror method. See:
FC90-sg2100.html
About 2/3 of the way down the page. If you do this mirror thing, and point the dish at the sun by hand, and move it around watching for where the mirrors reflect the sun, you can get a pretty good idea of where the focal point is. The problem in your case, is that the actual surface of your dish is inside the fiberglass, not at the surface, so the actual focal point will probably be a bit closer than the mirrors predict, but again, it will get you close enough to get a signal, and then tweak the FL a bit while peaking on the signal.