It was called an HT-2 test kit (Hypo Test Kit) They had a corresponding ruler-the hypo estimator-- that has a protective sleeve as well. There are stain densities on this ruler that correspond to test target stains listed in the Kodak B&W Dataguides and other publications & tech sheets. Different papers are broken down to different densities..you lay this ruler alongside the paper base you're checking & evaluate the stain visually. This test is not 100% positive. It's not as accurate as a test that would use a densitometer or other type of lab equipment. When it comes to processing of microfilm for example, in archives they use different tests to check for residual fixer, and these are usually performed at set periods of time, like weekly or at intervals through certain projects. The tests are done to comply with standards in use in archives & records storage ... there's also a test for residual silver that would check the activity of the fixer through a stain on the paper.
It could take years & years for any problems to pop up from incomplete washing. It might not really be a factor depending on how long you intend to use the prints, but whatever is left behind, can cause problems if the temps & humidity of storage get to a point where they can trigger this deterioration--if you stored them in a place devoid of any moisture or heat , they might fare better....