Where is the mint stamp on an old gold coin?

To set things straight, it's called a mint mark. In general, mint marks were on the reverse side of older gold coins but not always. Also, coins struck at Philadelphia did not carry any mint mark in those days, so a coin without a mint mark is perfectly OK.

I'd suggest looking at the site www.coinfacts.com. They have every date and mint mark listed, along with a description (and often a picture, too) of the mint mark location.