http://www.ncaa.org/health-and-safety/policy/frequently-asked-questions-about-drug-testing
From the link:
When is drug testing conducted at the championship?
Drug testing can occur at any phase of an NCAA championship (e.g., first round, second round, quarterfinals, semi-finals or finals).
Drug testing can occur more than once at any championship (e.g., first round and finals).
Participating institutions and student-athletes are not given any advance notice that drug testing is being conducted at the championship.
When will student-athletes be notified of their selection for drug testing?
At individual/team championships (cross country, fencing, golf, gymnastics, rifle, skiing, swimming & diving, tennis, track & field, wrestling), official drug-testing couriers will notify student-athletes of their selection for drug testing.
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The NCAA and institution also conducts year-round testing.
So to answer your question pertaining to wrestling, I don't think doping is very prevalent as the reward is far less than the penalty. The penalty is to lose a year of eligibility on first offense. Second offense is loss of all eligibility. That's a potentially steep price to pay (in scholly money) for the non-financial benefit of being an AA or champ.