There seems to be a lot of curiosity and confusion regarding drug tests, specifically for Marijuana, at DUI checkpoints. Some of the more enthusiastic bloviations showing up (in the always-factual message boards and comments sections of otherwise reasonable news articles) are even claiming that it is not possible to test for marijuana in a checkpoint or after an accident at all.
LAPD officer Kamaron Sardar demonstrates in this video demonstrates the swab test that will be more aggressively deployed in Los Angeles thanks to a $520,000 grant awareded to the LA City Attorney's Office.
The swab can identify THC (the active ingredient in Marijuana) for up to 3 hours after ingestion.
There will undoubtedly be more confusion and 'internet expert' theories and rumored loopholes (will it work with marijuana 'beverages' or only if smoked? etc) but even regular folks can learn more from any number of test companies.
Medical Health Tests is a good start.