When I bought online from Zenni optical they sent a plastic ruler that you set on the bridge of your nose with a slot in the middle, and can be used to measure PD. It's similar to this: https://store.eyeque.com/eyeque-pd-ruler.html

I found that the best way to get a nice, precise measurement was to take two pieces of sticky note and punch a hole in them with a pin. Put them on an existing pair of glasses and line up a distant (unless you're getting reading glasses) sharp corner with the pinhole over one eye. Switch to the other eye and see if the pinhole is lined up—if not, adjust the pinhole positioning and try again. Once the pinholes are aligned, you can measure the distance between them.

Much easier than trying to get someone to measure it accurately with a ruler in front of your face, or trying to draw dots with a washable pen in front of your eyes on old glasses, or any of the other methods I tried. The measurement I took appears to be accurate (the glasses I ordered from Zenni using it are comfortable to wear), and it was easy to get a precise and repeatable measurement to within about half a millimeter.