My limited experience with multi-accounters has been that it's not win% they're interested in as much as racking up quick points so that they can play in the "big boy" games. On the plus side, they aren't multi-accounting in high point games (because it's enough effort to inflate one account, let alone several), but on the minus side, it means that high point games may not be as competitive as they should be, with multi-accounters with weak skills getting into the game.
In detecting multi-accounters, anonymous games have been mentioned, but most of the multi-account games I've seen have been unannounced private games, password locked to keep out honest players who might report on the shady business. As such, there may be more multi-accounting going on then we're aware of.
I now check my opponents' previous games, and if I see any previous games with a lot of the same "opponents", or with a lot of one-game-only "opponents", I will report them.
Another serious problem, but one more difficult to deal with, is what I call the "club": a group of players who know each other well, often play in the same games, and give each other preferential treatment. That stinks in my view, even if it's technically legal, and even if some of the otherwise skilled players do it. The only real solution for the "clubs" is to put them on a personal blacklist, as I'm not sure there's anything else that can be done about it.