I agree it can be difficult to judge by eye, particularly since the stain is proportional to silver density. It should look somewhat greenish-yellow in comparison to a non-stained FP4 negative. FP4 stains quite well in PMK.

The conventional wisdom is acidic fixers reduce stain. I always used neutral/alkaline fixer with staining developers, but never tried a side by side test with a standard acidic rapid fixer, so I don't know if this is true or not. I just went with the recommendations in The Book of Pyro.