Regarding your first question, indeed you wouldn’t know beforehand if there are any negatives. [Though it is reasonable to assume most places with more than a few reviews will likely contain some negatives.] Since you know there might be loshon hora among the reviews, you ought to refrain from reading them.
So, I also take it then that you’re not allowed to ask a friend about a restaurant, a store or other service provider, since they might give a negative review. Have you *never* asked someone before going to a restaurant, a doctor, calling a plumber or a contractor, hired a photographer or a band, or *anything* else?
The Wolf