I haven't opened a Professor X, but assuming they didn't alter the construction from the SDCC suited Destro (and I think that's a safe assumption, since everything else about the figure is identical), the shoulder joint is the standard 25th-and-on style. I'd say you've got a 99.9% chance of finding that to be the case if you open one.

Thanks for the info, man. I didn't know what the sdcc roc destro had. I just checked yojoe and it says the roc sdcc destro had the same arms (but not torso) as the sdcc cobra commander, which was an Indiana jones figure.

Ah; I had forgotten about the Cobra Commander sharing those arms, too. The Destro (and presumably Professor X) arms do have the larger disc on the joint where it inserts into the shoulder like other Joe figures. The arms from the SDCC Cobra Commander are identical except that the post on the shoulder joint is the tapered, narrower style found on Star Wars and Indiana Jones figures.