John Hawks posts about gene doping, and how difficult it might be to detect:

The essay includes a few paragraphs that describe the prospects of future detection of gene doping. It may be very difficult. Detecting some synthetic performance-enhancing agents requires the cooperation of primary producers, who add tracers to their products. Gene doping might enhance performance for periods months or years after the vector is administered, and may not require dosage that would significantly alter isotopic or chemical signatures, even if they contained such tracers.

Again, maybe it would be better just to make this sort of thing legal and transparent, rather than wasting money trying to detect and outlaw it.