Do you want to detect the residues left on the fruit and veg, or the residues in the wash water?

Either way you will want to take multiple replications (at least 3, preferably 5 per treatment. You will need to include about 250g of produce per sample, both before and after washing.

Make sure you send the samples to a reputable lab and send for a multi-residue test (GC/LC/Dithiocarbamates). 

i think you can simply compare the before and after treatment calculations.

It depends which type of method you want to use, for quantification HPLC are used