Rh can be fused with sodium bisulfate (heated strongly till dry metal powder and salt (NaHSO4) become liquid like syrup), this fusion is then dissolved in water (do not use too much water), the rhodium will then be soluble in this solution, and can be tested for.
(See Laser Steve's test for Rh).
Very fine Rh powder will also dissolve in very hot concentrated sulfuric acid, and can be tested for.
I would use bisulfate fusion (easier and safer).
The bisulfate acts as the sulfuric acid in this process.
The sodium bisulfate is a byproduct of homemade nitric acid (white salts we precipitate) from our home made nitric (when excess H2SO4 was used with sodium nitrate fertilizer),
