You could try to quantify immunoglobulin levels in all samples using anti- immunoglobulin polyclonal antibodies for capture and detection (perhaps combining anti-light chain and anti-heavy chain to make sure both of them can be detected). Even though these levels can vary from one sample to another, you could compare the whole panel of old samples with a set of fresh samples, at least to detect gross problems in old samples.

Alternatively, you could try to detect heavy and light chains in western blot to assess that the bands appear in expected positions, indicating that the proteins are not degraded or aggregated.

A third option is to test them against an antigen that is universal (every human having an antibody response against it), perhaps a vaccine antigen used for many years, but you would have to find one that is highly immunogenic in every individual in the population you are analyzing. I dont know if this is actually possible.

Thanks so much for your answer. We are actually thinking of doing your third option - using a combination of tetanus and smallpox depending on the age of the individual and time. We have a limited amount of material that represents a valuable resource so trying to minimize use.

Almost everybody has anti-DNP or anti-TNP antibodies.  These are "natural" and probably not specific for those nitrophenyl antigens.

You can test them with DNP-BSA conjugate.  When doing that, ensure that you have sufficient concentration of BSA in the test solution to block any BSA-specific antibodies.