Resistance could be defined as their ability to grow in the presence of particular agent. You could use a range of drug concentrations and calculate growth curves for particular lines grown with/without the drug
You can do an assay like those performed to test the IC50 (inhibitory concentration for 50%of the cells) using a wide range of concentrations of your drug. You can follow the growth of your cells with colorimetric assays using MTT or Alamar blue.