TL;DR Graves at all are a good suggestion, worked stone is a pretty big hint, worked metal is hard to ignore, dismissing plastic would be silly, but finding medical implants would have to be totally suppressed to make a theory of unintelligence plausible.

Some few other species take interest in their dead so the future people might be forgiven in placing "true intelligence" above some higher bar than just organized dead.

Modern graves often have casket liners of metal, plastic or cement, these are obviously worked materials requiring advanced logistics to create. Possibly this could be dismissed as the work of some other group potentially long posthumously.

Similarly grave goods like the metal and plastic parts of clothing should survive, and be very clear indications that the site was prepared by an intelligence. Whether that intelligence was the creature in the grave could still be doubted.

Objects in the graves intermixed with the remains would much trickier to explain. Fillings in teeth beside unfilled cavities imply advanced care during life because postmortem decoration would presumably get them all. Clearer but rarer; healed surgical wounds especially those involving permanent metal screws or pins would strongly imply contemporary materials and techniques.

Nuclear powered pacemakers and other non-natural radiation sources from medical treatment would be a huge giveaway if found, but are rare enough that it might not come up.

TL;DR Graves at all are a good suggestion, worked stone is a pretty big hint, worked metal is hard to ignore, dismissing plastic would be silly, but finding medical implants would have to be totally suppressed to make a theory of unintelligence plausible.

Some few other species take interest in their dead so the future people might be forgiven in placing "true intelligence" above some higher bar than just organized dead.

Modern graves often have casket liners of metal, plastic or cement, these are obviously worked materials requiring advanced logistics to create. Possibly this could be dismissed as the work of some other group potentially long posthumously.

Similarly grave goods like the metal and plastic parts of clothing should survive, and be very clear indications that the site was prepared by an intelligence. Whether that intelligence was the creature in the grave could still be doubted.

Objects in the graves intermixed with the remains would much trickier to explain. Fillings in teeth beside unfilled cavities imply advanced care during life because postmortem decoration would presumably get them all. Clearer but rarer; healed surgical wounds especially those involving permanent metal screws or pins would strongly imply contemporary materials and techniques.

Nuclear powered pacemakers and other non-natural radiation sources from medical treatment would be a huge giveaway if found, but are rare enough that it might not come up.

A point was raised that all of this might be true in a pet cemetery as well.

The best evidence that the interred are the intelligence might be that the hardware of the coffins and the fasteners for clothes are good fits for human hands. But the actual tool marks on them would likely all be machines.

Work related bone growth might also suggest humans occupation. 15th century longbow archers have been identified by skeletal deformities from using heavy bows, possibly back or neck problems from computer hunching or tool marks in bone from industrial accidents or murders could give hints to modern life.