What does one look for to recognize a Pre 1900 W W Greener SxS as a High grade firearm. Extensive engraving? Marks on the watertable? Patent number?
WW Greener guns were built to a defined grading system, and the best way to understand that is to buy Graham Greener's book. As stated above, many guns, but not all, will have that grade engraved on the trigger guard tang. The advantage here is that if you can identify the grade, you know where in the spectrum of grades it falls. I'm not sure where to define that with the respect to the term "high grade" but I guess somewhere starting at the upper end of the mid grades (which were already expensive guns), so starting at FH35 or 40. These grades, at least at one time, represented the price of the gun, so a 40 cost more than a 35. But real high grades start with the G-grade guns and above in my opinion, and I believe they started after the FH50. Lastly, like many gunmakers, the level of engraving decreased over time for a given grade (I have 3 FH35s and there's a noticeable change from the earliest to the latest (about a 25 year span, ending in 1914 for my 3.
Beyond that, I'd look for features like:
1. Engraving coverage (although there are exceptions)
2. Shaping of the action fences, especially scalloped or arcaded fences or additional beading, also any reinforcing bulsters under the receiver balls
3. Wood quality (grain)
4. Ebony inlays in wood behind action
5. Engraving on barrels (not the top rib, but the breach end of the barrels.)
6. Toe and heel plates for the butt
For the vintage you're talking about, a customer could order whatever they wanted, so the world is full of exceptions.
Lastly, while Greener made all types of guns from economy singles through heavy double rifles, remember that WW Greener marketed the facile princeps design (which looks like a rounded boxlock) as superior to the sidelock and Anson & Deeley boxlock, so high grade FP guns were among their highest quality offerings.
I love Greeners cause they're a little different. Not rare, but different, and consistently good quality.