One method could be to find a relationship between all those systems and some common system such as the A, B, C, D rating system. There are a few threads about the letter grade system based on the likelihood that a player will run out with some number of balls on the table, or perhaps the maximum number of break & runs or runouts a player has.
I've been in a few tournaments in which a fellow player vouched for a newcomer's ability. At other tournaments, the player simply talks to the league director and they settle on a provisional rating. It may be hard to do much better than that on short notice. One option could be to show up early and shoot a few racks with the tournament director watching or playing.
In any case, it could be hard to spot someone who is sandbagging.