I read that donating blood can't be done if you are in the contagious stages of herpes. but how do they know that you have symptoms or in that stage, especially if for women who may have soars hidden inside?

Hmmm....: Haven't heard of that donation restriction. Perhaps it means a donor should not donate if he or she has any known active herpes lesions. That's acknowledging the fact that some people have a lesion they don't know about and thus wouldn't know they might have a donation restriction. One can double check with the local Blood Center to see if such a restriction actually exists.