Unless a window is signed or there is a reliable written record, you can't know for sure who made a window. However, during the search, there is much to learn.
Some studios never signed windows. Lawrence Saint said he would sign a design but not a window. Too many artists worked on it. Perhaps some studios considered stained glass only a craft and not worth signing.
The custom, if a window was to be signed, was to use the studio's name. Perhaps the glass painter whose responsibility it was to add the information conveniently forgot. "Why should I sign the boss's name? He didn't make this window -- I did."
New, stronger frames that replaced old ones sometimes made it necessary to cut off or cover up a signature too close to the bottom of the window.
The first place to look for a signature is the lower right. Then go over the whole window before you give up. A window in Saint Luke's Episcopal Church, Baltimore, has a signature on the hem of Jesus' robe. A window in Mercersburg Academy Chapel by the Camm Studio has not only the studio but everyone who worked on the window and several members of the family.
The signature of the Tiffany Studio is usually acid etched from flashed glass and visible along the bottom border, probably because very little of the window was painted. Of course, it should be remembered that not every opalescent window is a Tiffany! Though that seems obvious, not everyone is aware of it.
If all the windows in a building were made at the same time and are similar in style, only one or two may be signed. This is the usual practice of Mayer of Munich.
In one instance, a restorer signed an old window as though he had made it. A suspicious and dedicated archivist uncovered this deception.
The search for a signature may require binoculars or a magnifying glass.
Another challenge is that some studios use quirky logos or monograms; Charles Eamer Kempe used a wheatsheaf from his family coat of arms, and, when the studio was taken over by his nephew, Walter Tower, a tower was superimposed on the original logo. Powell of Whitefriars used a little monk that could be elongated to fit a vertical border. Monograms are easier to read, but the seeker must know the name of the studio or maker.
Sometimes an undecipherable scribble will look like a signature. It is possible that a glass painter has added his mark. Joseph Diano, a glass painter who worked for both D'Ascenzo and Willet Studios throughout his long life, used to say he signed every window, but no one could ever find his name. Arnolds Treibergs, another Willet glass painter, was observed to introduce a Latvian star where there was none in the design.
The next obvious place to look may be the archives. Some congregations carefully keep detailed minutes of procedures and old programs of services. These are often found in a slightly damp box in the basement. Some denominations have historical societies that collect archival material from churches and keep that material in good condition.
Sadly, some archives are tossed out without being examined when a new pastor comes or when a closet is needed for a more current use.
Make a note of any dates. These may or may not be the dates the window was installed. Maybe the family that donated the window is still in the church. There is a chance they have kept diaries or letters.
If the person memorialized or the church was prominent, the next place to try will be the files of old local newspapers or publications of historical societies. Be ready for disappointment, because articles like this usually dwell on everything but the maker of the window. The readers were considered to be more interested in the member of a prominent family.
These reports may contain errors and omissions, such as a lengthy one from the Richmond Daily Dispatch, 1878, which describes the process and includes every memorial inscription and text and then adds near the end, "The windows were put in by H. W. Jenkins and Son from Baltimore."
Another lengthy write-up from The Independent, 1903, of the windows in Calvary Church, Germantown, describes each window in detail, including the painting from which it was said to be copied. However, none of the studios that made them is mentioned.
There may be an old book about the church that includes information about the windows. However, it is usually the desire to create such a book that contributes to the interest in the letters.
It is tricky to try to identify style. It is at best an inexact science. Following are some hints used by experts who will give an "educated guess."
All studios duplicated their own designs. The same design may be in a completely different size and shape opening. You may be lucky enough to have seen the design in the window in question before. This does not refer to paintings from "Christian Calendar Art" like Plockhorst's "Good Shepherd," which many different studios use over and over.
Some studios keep records or put out material that lists their commissions so you can find out if they were involved with your windows.
Some studios use a definitive feature like Kempe's angel wings made of peacock feathers. Sometimes a color palette will be distinctive, like Bossanyi's. Large studios like Tiffany's hired many freelance designers who also worked for other studios; Frederick Wilson, for example, moved to California to work for Judson Studio after working for Tiffany, and Maitland Armstrong opened his own studio after leaving the Tiffany studio. Experts are probably familiar with these well-known examples.
A knowledge of dates that studios were in business should tally with the date of the installation of the window. A knowledge of styles popular in an era like opalescent or pictorial, preferred color schemes and architectural features helps. Experts have seen many windows and many styles in many parts of the country, in buildings by different architects, and will not pretend to know when they don't. If you ask them for help, remember they need to see the window or very good photographs not only of the setting but of details like faces, drapery and borders.