I mostly agree with Adam here, except as a PLAYER I wouldn't particularly care to be the person asking those questions.
I would alert a TD to, "Hey their scoresheet is already filled out 6's and they are on game 2, you might want to check that out". Let the TD have those conversations with the players, not another player.
Now as TD, I'm pretty liberal when it comes to collusion because I have to deal with it EVERY F*CKING YEAR at IFPA
I won't act on any hearsay from anyone that comes and tells me they saw something happen, or overheard something going on. I have however in the past gone and examined scoresheets during session 8's at IFPA's in the past when I've been alerted to possible collusion, and so far all the sheets I saw matched the scores that were on the game.
There's 3 instances where I act:
1) I personally see a scoresheet filled out with incorrect scores after verifying it with the machine in question. Adam is right that it could be an honest mistake, and if the group plays it off as that, I'll allow them to adjust the scoresheet to the correct actual scores. If any of them are stupid enough to admit to any collusion, they are kicked out of my tournament.
2) I personally see someone tilt with the ball in the plunger lane, or do a plunge/walk off as not player 4. If you want to tank, go ahead, just make it look good and I won't care.
3) I personally see a scoresheet filled out ahead of time, in which case those 4 players are kicked out of my tournament.
I have a real hard time with CHEATING in terms of not writing down the scores of the game that actually happened. I have a much easier time with people actually playing the game in front of them, writing down the scores of what actually happened, and moving on from there.
I also have a real hard time with BLATANT tanking that I'm personally witnessing as a TD.
Outside of that, go to town with your sandbagging, collusion, drug use and anything else that floats your boat (with respect to me as your TD).
Also as a player, this is your warning that if I'm ever your TD, and I ask you a question about whether you intentionally tanked a game . . . just say "NO" . . . and you win. Even if the other 3 players in your group tell me that you did, only your own admission would cause me to kick you out of my tournament.