If you need to monitor a fan on a piece of equipment this PC Fan Failure Alarm from PCB Heaven should fit the bill. The RPM monitor line is used to determine what the RPM of the fan is and if it falls below an adjustable threshold the alarm is tripped. Heat buildup due to a faulty fan can do some serious damage in a short time, this simple circuit could save you some headaches. The schematic is provided here so you can build your own.