Today's silly question: How do they know the 1895 earthquake was a magnitude 6? Did they have the instruments to measure quakes back then?
Strictly an estimate based on anecdotal and physical evidence.
Richter Scale came about in the 1920s..before that there was instrumentation (there are extant seismograph traces of the 1906 SF earthquake)....so for 1895 I believe there are actual records.
For quakes even older than that, like the 1811-1812 earthquakes, magnitudes have to be guestimated from a combo of maps showing the range of human "felt" reports and damage to structures (the Mercalli scale, in roman numerals) and physical evidence (how far a fault moved, how far away from the quake you have things like sand blows - where the shaking causes waterlogged sand to spill out on the ground from below, etc.)
These estimates are difficult and uncertain and you can get a lot of argument.
Interestingly most recently a lot of authors have reduced the magnitudes of the 1811-1812 New Madrid quakes from a lot of the really high estimates earlier; some have none of them being bigger than Magnitude 7.5.