Without the aid of mirrors or shiny metal (such as I believe some Victorian nobles used), the only reflective surface I can think of is a pond. I assume, though, that not all cave-men would have seen themselves due to, for example, only ever seeing running water. Is this a fair assumption to make?
Consider asking /r/AskAnthropology.
Seconding the suggestion for /r/AskAnthropology - 'cavemen' are far into pre-history. Meanwhile, you can get some ideas for other reflective surfaces (still water in a lake/puddle/bowl/cupped hands), smooth shiny rock like obsidian) from these posts