Well the larger "makers" actually had different serial numbers for different grades (Holland and Holland, W. W. Greener...) so if it falls into the period when that was practiced by those makers then it's easy with the right references (e.g. Brown). But usually of course it isn't so easy. Engraving sometimes corresponds to grade but I've some excellent guns that have nothing but fine border engraving (Horsley was typical here). Fit and finish is the best way to judge but one must have the gun in hand to assess that. Even then the interior may be better for the same level of outer finish. A good rule I think is is any one aspect is "coarse" whether it's the engraving - the choice of wood (grain not necessarily figure), the mechanism then it's a lower grade gun.
Don't rely on just the name. I have seen identical guns, including the engraving, (mid grade) retailed by different "Names" sell for different prices at the same time simply due to the name.
In my limited experience, third grade guns did not receive much engraving. Second grade guns can be as ornate as first but the fit and finish is not quite as fine.
Jeremy
