i use to have to do random saliva tests for a job i once had. it wasnt really a "professional" test. they came in packages and you open it and it had 2 parts. one was the cotton thing you stick in your mouth and the other was the part you stick the cotton thing in and it gave results right on it. but they give you a lil cup of water to "drink" before you stuck the cotton in, and all you had to do was keep some water in the jaw part of your mouth under the tongue and let it soak all the water and bam, pass every time.
your test could be different though.