When I find a player who seems to regularly cheat, it is usually in my local minis -- though I have never had a sanctioned tourney round with any of the guys I know of who cheat. I once watched an experienced player who was having a tough day four-putt in a mini (3 misses + the gimme) and the next tee box was literally 25-30 ft away, and he called out "4" when asked.
But here is what I do. Once I see that they've "forgotten" their number of strokes (usually by one or two less than normal) more than once suspiciously, I begin getting under their skin. From then on, I'll call out every shot they take as they begin to address marking their lie. "This is your second, correct?" loud nough for everyone to hear; then I ususally get a first look like "Yeah, alright, OK, enough." Then whatever happens, I keep doing it. "This is your three now -- we agreed it was two last time." "This putt is for four" "The gimme hole-out makes five" and so on for every shot on every remaining hole. If they tee-off OB, I'll say "you get up to one meter in, shooting three," again out loud so there is no mistaking what I am doing.
And if he says, "how'd you like it if I did that to you?", I typically reply with, "no problem...I'll do the same for myself." And then I do.
And for those who pencil whip when they have the card, I do what a lot of people said, I keep my everyone's score on an online app, and MAKE SURE to do the comparison at the end of the round. And if I suspect he pencil whipped when he scored, I usually ask to see the card for comparison two or three times DURING the round.
I would call you for a courtesy violation if you tried that. That is annoying head games, if you need to count my score do it in your head and double check my math at the end of the hole.