You will know when you've found it because the person it belongs to will become more aroused. Just try not to be too rough and do not apply pressure directly to the glans. It is very sensitive and most women find direct contact too intense. Try stroking the hood around the clitoris instead. And you may not get an immediate reaction.
Some women take a moment or two to warm up.
You can also ask a woman to lead you to the right spot, or even show you how she masturbates for a clue. While I don't know of any websites that have pictures of the clitoris, any decent health book that has topics on reproduction will have a diagram. Most women's health books, like Our Bodies, Our Selves , will have a drawing. And almost every sex manual has a drawing identifying the general location of the clitoris. Alexis Williams has a Masters Degree in Public Health from Emory University.