Hi, no specific marker to recommend. But... it's hard to beat the ID system commonly used by photo labs since as far back as I know. They're called twin-checks (or triples, even). These are adhesive labels that come in rolls, and have pairs of sequential numbers. They're designed to be impervious to processing solutions. In use, the photofinisher sticks one on the film (on the leader of 35mm film, for example) and one on the order sheet , or whatever. So the two are now linked by a common number.
If you have a local lab you might stop in and ask if you might purchase a few. Since most lab people are pretty cool folks they'll most likely just give you a foot or two. So, you write in a notebook as to what you are testing and attach one of the twin checks. The other, you stick on the back of the film or paper BEFORE it gets wet.
Another ID method, for a small number of prints, or whatever, is to use a hole punch (kinda like a pliers, but with a hole-punching apparatus). Change the number of holes for a sequence, or use some sort of coded sequence. A variation on this could be to make pinholes with a thumbtack (don't poke holes in your finger).