I would like to ask to this question: is there a testing procedure to figure out whether one has over-washed?
First of all, "overwashing" is a rather paradoxical description.
To make the story simple, I'll tell you when you have to worry about it and when you don't.
The notion of overwashing is completely irrelevant and unnecessary if (1) a pictorial camera negative film is processed; (2) your print is toned in any of the archival toners; (3) if you don't care about archivality of the print.
Overwashing can harm the image stability if you intend the untoned print for long term storage. Again, anyone who's sensible would use anyy of the archival toner for prints made for long term archive.
The notion of overwashing came from the finding that a very small amount of residual thiosulfate can sometimes protect image silver when there is no other protection (e.g., toning). However, the degree of protection provided by even the optimal amount of thiosulfate is smaller than a light toning with polysulfide or solid toning from selenium. Another problem with relying on the optimal residual thiosulfate is that it is impractical to aim the optimal value by adjusting the washing method. Therefore, anyone serious about archival printing should consider using any of the archival toning process, thereby making the notion of "overwashing" is irrelevant.
Manufacturers know that "overwashing" exists and it is counterintuitive to average darkroom workers who are not serious enough to go through the toning process. This is a very tricky issue in phrasing usage instruction, and each manufacturer thinks hard to be as "informative" and "courteous" but without making the instruction overly complicated. One example is that many manufacturers overly emphasize it is unnecessary to use more washing time than indicated. They may also emphasize that there is no need to use washing aid for RC prints as long as the fixer is fresh. They would still recommend polysulfide, selenium or gold toner if you are serious about image permanence.
Back to your original question. You can use methylene blue test or amylose iodine test described on the page linked below to test for overwashing, if you really want to do that. But as you already saw above, avoiding "overwashing" is more of prevention of wasteful effort than an effective and active means of increasing image permanence.
And more generally speaking, besides the test of time and the HT-2, what tests are reliable indicators of a properly processed print?
Described in this page:
Dead Link Removed
There are other tests that are intentionally omitted in this page. Those tests are unreliable or unsuitable and are currently not recommended.
My own understanding so far would be that although there are theoretical guidelines regarding wash, mere conformity to these rules does not replace actual verification, and does not warrant by itself that a print or a film was properly processed, right? Somehow I think that people can get by with a as little theoretical knowledge as is needed to validate a bullet-proof testing.
If you stick with standard processing sequence from Ilford, Kodak or mine (silvergrain), and don't deviate from the recommended usage and processing capacity, users can rest assured that their material is properly processed.
Testing is recommended if you have reason to be concerned, such as unfamiliar processing chemicals, unfamiliar paper stock, need to limit the washing water to the minimum due to water supply situation, etc. It is also easy and simple to perform if you are very serious about archival processing.
More often on this site people recommend to test for your own as a way to end otherwise endless battles from people with strong dogma. Think about it, most Kodak users, most Ilford users, almost all other darkroom users, they never run their own tests but you don't hear about images that are deteriorated due to too much residual thiosulfate from careful darkroom workers who recognizes the importance of proper washing. This is because the majority of deterioration of image silver is due to oxidative attacks and not residual thiosulfate. Most people who are reading this thread and concerned about image permanence are actually better thinking about the toning process rather than washing process.