You can test it with a short strip of exposed film, but that test won't give you any useful information. It will simply tell you that, yes, it will reduce the silver halides to metallic silver. It will give no indication that the developer will perform as expected. Case in point, and I've done this so I know it is true, is to take some previously used dilute developer and test it with a strip of film. It will darken the film, but will it perform the same way it did on the first use? No, in most cases, it will not. If it is not too far gone, I'd need to compensate for the reduced activity by prolonging development in order to reach the desired contrast index. Then the question is, by how much do I compensate? And the answer to that is subject to a whole range of variables that must be taken into account. Bottom line is that you need to test it with actual image bearing film in order to determine if it will work to your expectations. Would I bother? No. Developers are relatively cheap compared to the cost of film.
