The two-way mirror is probably not something one should worry a lot about, since a quality two-way is rather expensive. One can get "novelty" mirrors that exhibit "usable" two-way effect under "magic show" conditions fairly cheaply, but even a casual look in any ordinary environment usually reveals their presence quite readily. Anyone using one of the "cheap-o" mirrors for voyeurist purposes would run a very high risk of being detected, which tends to take the "fun" out of it, especially for the voyeur.

With the incidence of "shoplifting" so high, there are a few reputable places that purport to have "surveillance cameras" in place, including in fitting rooms. If the real purpose is to be able to observe the occasional "suspect" customers, such "cameras" are generally placed very conspicuously and will not be undetected by even the most casual patron. (About 99% of the time, the "surveillance camera" is nothing but an empty cardboard box painted/printed to look like a camera - for "deterrent effect.")

Judging by a few spam ads, and without additional research, the "method of choice" for the modern voyeur would be one of the many "spy-cams" that are advertised as "tiny and undetectable." Anyone with any experience of video photography will be aware of the critical need for very bright lighting to get useful pictures, especially with small (inconspicuous?) apertures, so one must doubt that these work very well. But - if you're skin peels before you can change the outfit - someone may be attempting to light things brightly enough to use one.

In the meantime - smile into the mirror (and listen for the "click").

John