You can assume that organic molecule are small compared to your liposomes (about 1 nm against 100 nm or more depending on the size of your liposomes), If diclofenac sodium salt is water soluble it will be entrapped into the aqueos volume inside the liposomes, so you can use size exclusion colums made of sephadex gel to separate the loaded liposomes from free diclofenac.

Go for centrifugation you get two layer 

1.supernatent

2.pellects

1.supernatent contains unentraped drug mens free drug 

according to argument you get your free drug. do GPC of it you  get your free drug.

In addition, you can try to filter your formulation from a hydrophilic cellulose acetate membrane filter.