The isotopic check...not rocket science but maybe a bit tough to explain.
Let's put it this way. Use of very sophisticated mass spectrometers can
give you the exact weight of a compound...and identify the compound by virtue
of the weight of the fragments that the compound break up into.
What this means is that with a great mass spectrometer, can also check the
ratio between 12C and 13C atoms that are in the molecule.
In Landis' case, I'm GUESSING that this ratio was taken for Landis in the previous
stages...and that it is different in this one sample...thus thinking that
Landis' has some "added" testosterone.
I know that's probably confusing, but in the right hands, these ratios can
do incredible things. Such as finding out the region of England that a
pre-historic man lived in, because we can compare isotope ratios of
his dental material (from his skeleton) and match it to isotope ratios
in the ground water (I can't remember for sure if that was a check on calium
isotopes, but I saw it on PBS's "Secrets of The Dead").
I also think it's used to find out if ivory has been harvested legally (from
a certain region) or illegally (from another region).