[Wonder Warthog]

" Tag the fracking liquid with short lived radioactive tracers, like the same kind of radioiodine used in medical procedures."

Short half-lives won't last long enough to trace the flow. We're talking feet per year (or maybe inches per year.

"Or some harmless chemical substance not found in nature. Then get samples of the water table water to check for the tracers’ presence."

Fluorocarbons. Soluble in the oil. Available in both liquid and gaseous types. Detectable at incredibly low concentrations with electron capture detectors. And harmless to the environment.

[HiTech RedNeck]

Fluorocarbons... I can just imagine the greenies screaming “Freon!”