You could try to implement a blind test by doing tDCS with currents below those known to be effective as a control. From what I've heard under 0.5 mA should have negligible effects, though its worth looking into more studies for that. You should also implement some sort of double-blind by having another person set control the Amps for you. It wouldn't be too hard to design an experiment around this principle. I would check out some of the stuff on http://gwern.net for reference on designing quantified-self experiments to establish an effect strength.