yes its easy. you can use centrifugation to separate nanoparticle-drug and free drug. If your drug has absorption in Uv or visible region then you can prepare a calibration plot with drug solution only. Then after Nanoparticle coupling, use centrifugation the nanopartcle-drug will settle down at the bottom of the centrifuge tube. separate out the supernatant. redisperse the nanoparticle-drug complex in an appropriate solvent. Moreover, measure the UV-Vis spectra of supernatant and find out the concentration of free drug in the superanantant using Calibration plot. This way you would be able to quantify that how much drug has been attached to the nanoparticle surface.

you can also use ultrafiltration centrifuge method , in this method you use a Microtube with a filter that the free drug can pass the filter and you analyze its concentration