These players usualy suck less hard.
check out his stats on http://www.sharkscope.com, if he is a coherent winning player you should assume he plays decent. Usualy these players know about position, odds, ICM, ...
It is vital to make notes on multitablers. Write down how he play's his hand (coldcalls JJ, 3bets AQs IP, ...) because you will likely play against him again