Is there always sperm in pre ejaculation? is there any way of telling that a guy has pre ejaculated?

Pregnancy: Unprotected intercourse has a high risk for pregnancy and STD. If the man has not had a vasectomy, then the pre-ejaculate has a high likely hood of containing viable sperm.

Usually: Pre ejaculation or precum has variable amounts of sperms, which is difficult to predict. The other part of your question can be answered in a couple of ways on further clarification.Women can only 'tell' if a man has preejaculated if the fluid is excessive in amounts, which is rarer. In most cases women are more likely to tell ejaculation mainly due to the usual crescendo leading up to it.