A decade ago, a friend showed me how if one shuts down the electrical features of the dash of a 2003 Chevy Corvette, by removing a specific fuse, the car is drivable but the odometer no longer records the mileage driven while the dash electronics are inoperable. Is there a way to ascertain the actual mileage of a car that's been tampered with in this manner? Are there cars on the market where this type of tampering is still possible?
AFAIK, it's still possible on any car. Interrupt whatever mileage sensing system they have and the odometer won't increment. Older cars it was cable driven from the transmission. Newer cars have electrical sensors. Either way, interrupt the signal and no mileage gets counted. I can imagine it may be able to come up with a system less easily fooled if you take into account transmission gear, plus rpms, etc, but now you've decreased reliability... Trans gear sensors seem to fail pretty often (ever seen somebody who's had both backup lights out? Odds are the lights are actually fine and it's a failed gear select switch!).
It's illegal to tamper with that on purpose, but at least where I live, there's no legal requirement for the odometer to work. If the sensing mechanism breaks down, you DO have a legal obligations to report the guesstimated mileage to someone purchasing the car. Just as if you have to replace the odometer itself (gauge back in the day, or module that stores the value nowadays) you have to report the fact that you had xxx,xxx miles on the old one, a guesstimated value driven before repair, in addition to what's showing in the car.
Odometer tampering always has been and will probably always be possible. The only thing likely to change is the difficulty involved (and perhaps the legal penalties for doing it).
If the perpetrator doesn't thoroughly cover their tracks, you might find hints that something isn't right by checking service records and Carfax (or similar services). Oil changes and preventative maintenance at intervals that don't makes sense. Ridiculously low numbers of miles driven per year, etc. That's not going to reveal an accurate, 'true mileage' number, though.
The authoritative (and historically only) source for 'true mileage' is the odometer in the instrument cluster. Models within the last few years may also store a mileage in the Powertrain Control Module and / or a Body Control Module. You could access those values with a scantool if they're present.
For a C5, though, the mileage is probably cluster only.