Assume that the toxic compound is some form of liquid (water or oil soluble).
Take a sample of the bacteria and irradiate them (inactivate them permanently). Immerse the inactive bacteria in the toxic liquid for a certain amount of time.
Remove the bacteria later, "wash" them off (against a sieve designed to filter bacteria) for toxin traces that could be on the membrane or cell wall, then liquefy the bacteria and test for presence of the toxic compound. If the uptake is active, there shouldn't be any of the toxic compound in the bacterium.