claycrusher 900 wrote "... that English guns are graded, such as best, second, and third." He asked "What separates the grades, and how would a person tell what grade he is looking?"
I can speak accurately on behalf on one of the most inventive London gunmakers as I have studied his guns and have copies of his paperwork; the well respected Frederick Beesley.
Looking at catalogue descriptions the key contemporary separator that claycrusher1900 asks about is price. Thus a Beesley from 1922 could be had for 120 and would be 'best' in every respect, made to measure for the individual client and of superlative quality. Beesley called these guns his "Standard" grade. They were made by his own small team of craftsmen, though of course the locks and barrel tubes were bought in. Beesley himself oversaw this work and carried out a good deal of it himself.
At 75 come the Special Quality. This would be his second grade. He writes that the gun is 'A very high class gun to meet the requirements of those not caring to pay the topmost price, yet desiring a weapon combining selected materials, first rate work, fit of gun, balance and general finish, and is equal to most makers "best" quality.'
Then at 45 there was the Three Quality. This was his third grade. Again, it is best if Beesley himself describes it: '... of the same sound and reliable build as the "Special", but being made on the box action principle which offers facilities for the use of power in its manufacture, enables us to offer it at a price within reach of persons of the most moderate means.' These boxlocks were made superbly for Beesley by John Harper in Birmingham, then sent to the London shop for finishing and fit.
I have refrained from using the earlier late 1800s catalogue I have, since it goes into more detail on the many other types of gun Beesley could provide. I feel that the 1922 catalogue rather more succinctly answers the question about grades differentiation.
With regards to being able to tell what grade one is looking at, that is really a matter of ones own knowledge and experience, I feel.
Tim