It could be done with eddy current electrical conductivity measuring equipment. It could be done by density, if you have a jolly balance (used for rubber and plastic density determination). It could be done via SEM/EDS. But, I'm guessing that you don't have access to any of the above. You might try color comparison. Polish a small area of the part and compare to a known piece of brass and phosphor bronze, also polished. The brass will be more yellowish, the bronze more redish.