Photoplethysmography. Same idea, it's based on genital vasocongestion: which in men causes erection and in women it causes blood plasma to seep through the vaginal wall--this is the main constituent of lubrication. Instead of a strain gauge or volumetric device, it's a tampon-sized widget consisting of a light source and a photodetector. These work in tandem to measure a relative quantity of blood in surrounding tissue, and hypothetically, arousal.

There are some issues comparing the results of phallometry in men and photoplethysmographic results in women, however: https://link.springer.com/article/10.1007/s10508-019-01599-5 Should get you started if you want to know more.... but at this point... TMI? =)