One would most definitely know if its saliva and you know it isn't yours. Although they'd put a straw in your cup, one will have to wonder how thoroughly the cup was cleaned since it still smelled like someone's saliva.
Do the test yourself, there are several establishments that do not use disposal cups. Try it and prepare to vomit.
The worst offenders are:
1. two of the oldest pizzarias in the Philippines.
2. a roast place named after a musician