During which years has the photographer been active? Were the 30s the end of her creative period, or was this the beginning?
Just judging by the size of the print and the information with regards to time & the cameras, I´d guess that the two big prints were enlarged and the two smaller prints probably were contacts. It was very common to have different plate-holders for different sizes of plates depending on the particular needs of the shot (assuming glass plates were used, and not film).
Leaving the size of the print on the side, you can look for clues on what type of camera and lens was used and what modifications were made during printing. For instance:
1.) How is the colour reproduction, what kind of sensitisation did the taking medium have?
2.) How is the contrast, edge sharpness and the Bokeh? Does this look like and old Petzval taking lens, or is this a 1930s brand-new crips sharp Tessar picture?
3.) Look for marks of dodging and burning-in: Which techniques were used, did this happen on the negative / a mask or was it only possible with an enlarger?
4.) Can you see some defects at the borders (maybe under a passpartout) of the print which might be a sign of a plate holder or some coating defect at the border of a plate?
As suggested by Ian and NedL, grain and grain sharpness are good indicators, but I am not sure if a common magnifying glass will do. Maybe this requires some heavier equiment. Besides the interpretation of the results might be difficult as you don´t know which materials and tools were used.