The best you can do and hope for is that someone did at least a cursory visual check before vegetables are rinsed and during the act of rinsing itself.  We all need to keep in mind that slugs and snails are often quite minute, translucent, and easy to miss (and this article does a slight disservice by picturing slugs as large and easily visible).  

Now, as for whether slime left by the passage of a slug or snail contains any or enough of the parasite to pose a serious health risk, this is where true consensus among health authorities does not exist.  Some say yes, some say maybe, and some say no.  Most will concede that complete removal of slime from leafy vegetables is difficult, if not impossible.  That being the case, it's up to you to follow the experts or authorities you trust most.  Personally, if I noticed slime on any produce I thought of eating, it goes in the trash.

A real poser comes if you accept a meal from friends or relatives.  As the song goes, "Ya gotta have faith."