What a great idea! I just discovered this very clever way of of tracing connections on a printed circuit board on Instructables, from 2012. Rather than having to probe every single solder/contact point on a board, Instructable user “hertzgamma” starts out with a special fingertip testing probe, made out of a wire attached to a clip on one end and some aluminum foil on the other (wrapped around his fingertip).

Clipping this makeshift tester to a multimeter probe needle, he then places one probe on a contact point of the PCB and then tests areas of the board with his finger probe until he gets a signal. When he does, he simply switches to the normal needle probe and begins testing the contacts in that area until he finds the correct trace. Voilà. Easy-peasy, lemon-squeezy and why didn’t I think of that?

The above videos and description tell you pretty much all you need to know, but there is an Instructable that goes into a bit more detail. Also, see the comments on the Instructable post for cautions about capacitors, shorting and shock risk, and some additional tweaks to the design which could be useful.