As I clean them.. I wipe off any oil/residue with brake cleaner and a rag. The stains in the metal from the old gasket show right back up. Wipe til the rag comes back clean, then you know it's clean...even if it doesn't look it.

If you saw a clean manifold in my pics..it was probably soaked with purple power and then pressure washed. I'd do 4-5 manifolds at one time.