if they want to know if you smoked weed, they would have to run a specific test for drugs... a very different test that they would not run unless they had reason to.

ps- im a nursing assistant at a hospital, so i deal with this stuff all the time. my disclaimer is that if your parents ever got suspicious and asked him privately to run a UDS (urine drug screen)... then you are BUSTED! good luck in making a wise decision...