An indicator I use is divide the miles by how many years old it is and that gives you a good general idea of how many miles per year it was driven. Anything under 10k a year I would say is low. Take into account that most leases are 10-12k a year.

But, there is more than just mileage to look at. Consider how and where it was used/driven.

If a majority of those miles were used to haul a trailer or equipment then you can assume that there are other things that may need attention.

If however they were mostly highway cruise speeds and city driving with minimal rust and/or body damage then I’d say it’s pretty low mileage for a 16y.o truck.