Well I suppose you really cannot. My arms did grow a lot faster than any other body part, and I did not get any lethargy (of course there is 4AD in there).
I am guessing that it is localized because my arms grew at a fairly fast pace, but I think that Par Deus said that there will be some systemic absorption even though it should be pretty minimal.
Without actually having some blood tests done there is no way of knowing if there was any systemic absorption. Even though I did not get any of the side effects related to systemic absorption it could have been that the amount actually absprbed was not enough to cause any systemic side effects.
Either or I am not complaining.
I do see your point though. If the results are due to systemic absorption then it would be cheaper to just buy the S1+