I drugs used in hospitals. But while it is widely know that these drugs are illegal to distribute, posess, purchase, or consume, it is still done. To answer this question, yes. Government created search engines seek out those sites that are potential distrabution sites for illegal drug activity. Another method used is the programming aspect.
Some sites require a password to enter.
The employees that are hired hack into these sites to see if the activity is legal or illegal. Most of these sites are usually connected to people who are small-time dealers, and who sell to a small ring of clientele. The final method I found are government gophers. Gophers are those who pretend to be clients of a site, and see if it is serious or not. Gophers usually purchase more than once to be sure that they have enough evidence to convict an online dealer.