I have titles for vehicles that have 5-digit odometers. Some have said "exceeds mechanical limits" and others have an actual mileage number. In theory any vehicle that has a 5-digit odometer is supposed to say "exceeds mechanical limits" but if whoever owned the vehicle renewed their registration at over 100k miles and they wrote in the correct mileage, that mileage goes on the title.
The best way to try to determine what the actual mileage is would be to get a Carfax. As long as mileage was reported, it will show up on the report. I got a carfax on my one BII I had, it showed something funny with the mileage (but carfax never specified it as a problem). It all made sense when I pulled out the gauge cluster at one point and found a sticker confirming my suspicion that the cluster was swapped.... they swapped it with one from a 4.0L Ranger. But near as I could tell from the mileage that was reported and what the OD showed, I was able to determine that the truck had between 160k and 220k miles. Big spread, sure, but it was close enough for my purposes.
Part of the issue is that not all states require you to report your odometer reading when you renew your registration. PA requires the OD reading if you mail in the renewal, but if you do it online, they don't ask for it. Some other states don't ask at all, so there tends to be gaps.
And then you come to something like my F-150..... it has a 6-digit odometer (electronic to boot), supposedly had 69k miles on it when I got it. But it also had some strange discrepencies where things were worn out that shouldn't really have been in that low of mileage. I never bothered to get a carfax on it yet, but it makes one curious.