Now I'm sort of confused what you're asking. There is a huge difference between "snake oil" and prescription drugs. 99% of prescription drugs actually work. Vioxx probably does do whatever it's supposed to do, they just don't say what it does other than create random grassy fields and play soothing music. That 1% that doesn't work would be Paxil, quite possibly the most useless drug in history (opinion of course).
If you're talking about As Seen On TV type drugs, there is a way to tell which ones are legit. The bull**** ones never show the price, or the company address; they'll only show a phone number and will only accept credit cards. TV products like the Ronco Food Dehydrator will always show the company's address, the price, shipping cost, and methods of payment. Useless crap like Enzyte just shows a number and has credit card logos. Even that somewhat useful Calmax calcium suppliment clearly stated the cost of the product and how you can pay for it using cheques, money orders, and credit cards.
Also, anything that shows scenes in black and white with a red X over the screen is BS. "Are you tired of doing situps that just don't give results"; boom right there we know this product is crap. "No more starving yourself"; again, if it's black and white when it says that, the product probably doesn't work. A good product will sell itself. It doesn't need stupid zany tricks.