Is it true that semen stains look crusty in appearannce, while drool stains are just translucent, on like fabrics?

Very much so.  Ultraviolet lights (Black Light's) can also help examine crime scene's very well with these substances.

Semen stains are "crusty" looking while they are darker and less transparent than a normal saliva stain would be. If semen is present on a darker color fabric espically it will be pretty noticeable depending on the amount. Like qbp said, a blacklight would be very useful in locating the stains.

After working 31 years in a Forensics Laboratory on Sexual Assault Cases, I believe I can answer your question. Seminal stains CAN look crusty on a fabric but they do not always. That is why the use of the light source as a tool for finding seminal stains can be very helpful.I have handled many cases where the stain could not be seen with the naked eye but tested positive after detecting it. Drool stains meaning saliva can be picked up with UV light. I had a case where a baby was sufficated with a plastic bag, and I detected saliva using the UV light and than chemical testing. Hopefully this will be of help... Marianne:)