At some point, it's hard to be sure about anything. We can just guess about it. In your example, you said about coffee, and after receiving good reviews from others you decide to take the risk. Your taste can be different from others and you may feel that coffee is not as good as others said. But this risk was a calculated one because most of the time the result remains positive. It was necessary also because if you don't taste it then how do you understand the taste of it?
Indeed, being sure about anything can be problematic.