I don't imagine there would be anything conclusive for a variety of reasons.

By virtue of being prehistoric, it can't, by definition, be referenced by a written record. This means that any record of such a hypothetical pandemic would have to be based on residual physical evidence, which would be extremely scarce.

Pandemics are differentiated from other disease outbreaks in the sense that it has to be spread out and affect a large region. This means that to find physical evidence of disease spread over a large region, you would have to actually find multiple instances of that disease being spread over a large region. However, both bodies and pathogens naturally degrade over time so it would be extremely hard to find any physical evidence at all.

While it may be hypothetically possible for evidence of a mass dying caused by sickness (ex. while rare, a mass grave with preserved active/inert pathogens, perhaps preserved by cold conditions such as glaciers or permafrost could potentially be discovered), to assert that a pandemic had occurred, you would need to find additional evidence (other intact burial sites or other physical evidence) over a large region, which occurred at roughly the same time. An anthropological find like this would probably be worth a Nobel prize.

Herds of animals or human tribes dying out from illness is a pretty common occurrence in nature but geography and other natural barriers would've discouraged most pathogens from becoming pandemic. Diseases affecting animal populations tend to self-limit due to a variety of factors (such as geography, climate, vectors, predation, population collapse, etc.) so spreading a disease to the point that it becomes pandemic seems rather implausible in prehistoric times.