The worn out, chipping, shot-to-**** front bearing in mine felt perfectly smooth by hand. Only once the bearing was off the shaft could I feel how gritty the race was.
Chipped teeth on the gears can feel perfectly smooth too. Happens all the time with rings and pinions.
A groove worn into the countershaft would be impossible to 'feel', same with excess end-clearance of the cluster.
Synchros are impossible to gauge by feel alone. My bad 2nd gear synchro would still 'stop' the gear mid-shift when checking by hand, yet in the car it would give just a bit of a grind like I said before.
Removing the side cover is such a non-issue that if a seller won't take it off it's a big red flag in my opinion, and immediately makes it a 'core' IMO (assuming no invoices for a recent rebuild by a reputable shop, with photos of the internals).
