They can test for both, actually…

With alcohol, they can typically test for recent use by quantitative analysis of ethanol using a flame ionization detector (FID). They can also test for Ethylglucuronide (EtG) and Ethylsulfate (EtS), those are the metabolites, and are present in ones system for much longer then EtOH.

With marijuana however, there is no real reliable scientific way that can distinguish between someone who consumed it the night before at home and someone who just took a hit in the parking lot before clocking into work. At least not as far as I am aware.

Sure, you might be able test oral fluids and/or try to figure out an interpretation of the various ratios of all the THC metabolites in a system to try to draw a conclusion about how recently someone used. But again, not likely to be a reliable scientific finding, nor would it hold up in a court of law.

Source: am scientist-forensic toxicology