An ordinary phone toner and wand will do it. It's the method that's a revelation.

Here's what I do:
Connect one terminal of the tone generator to power ground.
Connect the other terminal to the shield of the cable you're looking for.
Check for signal with the wand. Don't want to accidentally leave this on "continuity"!
Go where you expect to find the cable you're looking for.
Spread out the cables across your hand, flat to the palm, with no overlaps or crossovers, with at least 1/4" between the cables. Since this test involves detection of minute differences of induced noise, you want all the cables to have the same range of capacitive coupling to your hand.
With the other hand, touch the jacket of each cable. The one with the loudest tone (and the lowest hum) will be the cable you're looking for.

A signal is induced from the wanted cable into every adjacent cable, its strength correlating with how far a distance they are parallel, but that induced sound will only be the same volume if they are parallel for a hundred feet or so. I think. I've actually never seen that. I've never seen that method fail.

Any cables that have been connected to ground will improve the ease of finding the cable you're looking for because their induced noise is grounded out. Cables not connected to anything will have a lower toner signal and usually more hum than the cable you're looking for.

Once, with this method (I feel like I should say 'at band camp' here), I could not find the kind of strong signal you'll get used to finding this way. I got very weak signals on several cables, but a slightly stronger signal with hum on one cable. I came up with the thought "this is acting like a cable that is next to the one I want, but is not connected to it. Let's see how far this goes."

I then popped a ceiling access and went into the attic... only to find two pieces of cable tied in a large loose knot. Whoever prewired ran out of cable and tied these together to finish the run, probably thinking to come back later. I connected them and easily found the other end.

The point there is that the cable I located exactly fit the description I'd come up with: I found "a cable that is next to the one I want, but is not connected to it."