firstly you should test an standard drug (for example Phenol Red) to know the normal permeability of Caco-2 cells to it. After that you can test several concentration of your tested drug with this standard, to know when the permeability is increased due to damage in your monolayer and to know which is the maximum dosis to be added without being toxic.
Measurement of TEER is also very useful to know when there are some disrumption in the Caco-2 cell monolayer.