Unless you buy equipment directly from owners, you'd never know who owned it.... Many pro-level cameras were owned by amateurs. On the other hand, amateur level cameras were usually not owned by pros. Pros also tend to own many bodies and not all of them were used heavily. Some pros take care of their equipment where as some do not. There are all kinds of pros.... wedding photographers, news photographers... etc, etc, etc.
I usually look at the equipment and try to guess what kind of lives it may had in the past. How pieces fit together, how buttons and knobs work, cosmetic damages, etc, can tell a lot. Just because something looks pristine doesn't necessary mean it lived a trouble free life either, but usually looks combined with basic function tests are pretty good indicators.