Growing up MUNI did have fare inspectors, albeit not too common and mostly on the light rail. However, I haven't seen a fare inspector in years, and fare evasion is noticeably worse these days.
There are inspectors, they always start out of Van Ness Station and ride out to the lines they are checking for the day, then back out Van Ness. Statistically, you are most likely to get checked at that station as a result.