pavementends, it does take a bit of math to do it but the process is this:
Insert the rod into the barrel until you have just engaged the rifling with the patch. Then put a tiny mark on the rod at as close to the 12 o'clock position as you can get. Then put another mark on the rod just as the patch emerges. Without any measuring equipment, you will just be eyeballing how much of a 360 turn the rod took. But, consider that in 3" barrel a 1:18.75 twist will have moved about 3/18.75 of a complete turn and a 1:16 will have moved about 3/16 of a complete turn. This is probably too small a difference to eyeball but then, it is probably too small of a difference to matter! I am starting to wish I had never brought the issue up!