Get a 3-wire fan from an old PC. Connect the tachometer wire (usually white) to the input pin. Use a 2.2K pull up from that pin to Vcc. Make certain the fan gnd is connected to the PIC GND. Power the fan with 12V (it may even run with 5V, depending on the fan). The fan will be a little "signal generator" for you. If the fan is running 2400RPM, the Pulsin value should be about 625, if you are running the PIC at 4Mhz.