I disagree with the test for film developer -- Film "should" be developed as consistantly as possible. If there is any chance that the developer is significantly weaker than it normally is, it should be tossed out. It is not just a matter of developing highlights to be as dark as fresh developer, but other factors as well -- such as mid-tone development and precise timing for getting the contrast one wants in the negative.
One can use old developer if the negatives are not very important -- but then, why spend the mental and physical energy in the first place if it is not "important".
I always use film developer one shot, and if there is any doubt as to the quality, it gets tossed, if one wants long lasting film developer, Rodinol and the HC developers would do it. If it's not important it gets shot using the d*****l camera
Prints are less of an issue, because you can always reprint, you can't always reshoot. Best test for paper developer, take a scrap of paper, under the safelight, put a piece of blank processed film in the enlarger, set the enlarger for an 8x10 sheet, and the aperture at F/8, put a black card over the paper, expose 1/5th for 5 seconds, then expose another 1/5th, so you have one a 5 seconds, one at 10 seconds, keep going until all sections have been exposed. Develop and fix. You should have a black section, and several grey sections. If it's all grey, then your developer has probably seen better days....