Hammer guns will not have the same clues to work with as a hammerless gun such as stocked to the fences, intercepting sears, or lack of a through lump. You really need to know a bit about guns from the time period, patents and the use of them in the British trade as well as a good eye. There are no simple answers. By sticking to well known makers you can certainly keep the odds in your favor but ultimately most every manufacturer, with the exception of Boss made a variety of quality levels. If you are unsure you can still contacts many British makers and see if they still have the order books. Many charge for the service of looking a gun up while others, such as H&H will read it to you no charge but only charge you if you want a copy of the ledger and a letter.
Steve