There are several different types of assessment that may be undertaken.
Evaluating artistic merit
The same principles apply as any other artwork. I don't wish to make judgements about this because it is so subjective, but I do enjoy a piece that uses metals and colours in a way that work well together so that one isn't dominant or discordant with the other elements of the work.
Assessing the quality of workmanship
I do have strong opinions on this subject. Bright gilding should be clean, clear and bright. Matt gilding should have a pleasant satin texture, not muddy or dull. Lay lines should be even and straight.
With lettering, there should be no gold residue on the outside of an outline. This is a definite no-no and is a mark of sloppy workmanship. The lettering should be backed up evenly and the varnish overlap should be of consistent thickness and less than 3mm.
The glass should be properly cleaned and have no gelatin streaks, residual pattern marks, finger prints or spots of free-flying paint.
Analysing techniques used
With so many different techniques available, it can be interesting and instructive to determine how a work was made and what steps were taken. By examining closely, you can often see the order in which different elements were applied. Examining the back can be very helpful too.
Studying the work of others opens doors to new methods and also can show if certain techniques fail over time. For example, red is notorious for fading on shop windows, but I have noticed that it will last far longer when it has been backed up with metal.
Establishing the provenance
When presented with an old unsigned work of art, it beckons to be investigated. Certain artists have very distinctive styles and techniques and are immediately recognizable. Most paintings are not that easy but the techniques may indicate a region or era in which it was made. The frame and back of an artwork often provides clues too.
Also bear in mind the glass itself. Float glass was developed around mid 20th century, so if it is float glass, it definitely won't be an original 19th century, Qing dynasty reverse painting.
Understanding heritage value.
The next article deals with restoration and conservation. Decisions to restore or conserve must be based on an informed view of the importance of the piece of work. Is the value due to its social heritage (an original sign from famous business)? Does it represent a particular historical event or time? Is it a good example from a certain artist or region? Does it use unusual or rare techniques that are no longer practiced?
When these things are known, the decisions regarding how to treat the work become easier because it's true value is better understood.