Unfortunately, in the interest of safety, if I had even an inkling that something I was to use in a potion was tainted, I would toss the entire batch and anything I may have accidentally used it in. I could not in good conscience, dream of using something I thought to have gone off or be unsuitable in any way.

I fear that was probably not the answer you were looking for, but in my professional opinion, the right choice.

I do suppose, pending the ingredient and what you think is wrong with it, there may be a test or two to try on it. It really does depend on what you think may be wrong with it.

If it was already in a potion, again, pending on its purpose, you may be able to test it somehow, but it really is circumstantial.

I'm not being much help, I'm afraid. Is there something I can take a look at for you?

Any number of things could cause a potion to change strength, most of which I am sure you know about. A brewing process mishap, contaminated ingredient, too much of one thing, not enough of another. Is there a particular potion or ingredient you are concerned about? I may be able to try a couple of things if you have a sample?