There are two methods for checking exposure timing in any Yashica E35 G xx; one is cheap, just buy 12 exp. roll, shoot it and send it to your lab. Later, check if everything is OK (I choose it as the simplest, least expensive and faster) . The other one is using an exposure meter of known accuracy, a special electronic meter to check shutter speed (or timing), which is not only expensive but also rare, and a source of light with variable intensity and stable colour temperature (only factory QA labs may have it). A third option is Jon Flanders method which will give you a very good idea of shutter performance specially if you have
dark areas which exposure time should be at least 1 sec. or longer.
Many 35 mm and MF film cameras, some weird cameras... and nothing digital yet!