By measuring labelled serotonin uptake in brain tissue-derived synaptosomes and testing the effect of SSRIs. In this case the reliability of the preparation, stored at -80° should be checked (I'm confident that it could work the same).
Alternately, the same experiments could be performed in brain slices (micro prisms).
Using a fixed serotonin concentration (just above the IC50) and increasing SSRI concentration the inhibitory potency can be quantified.
In case of interest, we could supply a general protocol we apply for uptake studies in synaptosomes and/or slices.