The flight manual, and pilots manual, gives guidance, but most private pilots, (myself included in an earlier life), have all overloaded singles, without even thinking about it.
In some flying schools the POH is hidden away, and a student or pilot practically has to sign the official secrets act to get hold of it!
IMHO, the whole problem starts in SOME flying schools (I'm not suggesting ALL flying schools). School aircraft are sometimes overloaded during training, since they were never designed to take two large people and lots of fuel. Instructors know they can get away with it if the runway is long, the weather is cool, and they have a headwind. So they do, not realising that in the absence of any explanation, students will copy what you DO rather than what you SAY. I asked an instructor about this way back when I was a very new PPL. He told me that although you really shouldn't go over the weight stated in the POH, there was usually some leeway. Just don't do it, he said, if you're at 2000 ft and there's no headwind and the weather is hot and the runway is short and you're uphill on grass...ie, don't push your luck. It was good advice, because it led me to understand these things rather than just following the rules...or ignoring them.