unless they've gotten more advanced, saliva tests typically can only detect weed for a few days. drink lots of water and use mouthwash before the test, I would recommend against vigorously brushing the day of - but the days before is probably helpful. I passed one once and had smoked a few days previous.
take a big swig of water and swish it around as close to test time as possible. Try not to sweat it too much, I think you'll probably be fine unless you've been smoking heavily / regularly.