Note: OP said IN the PSU. Most PSU fans I've seen have only two wires for their 12v dc power supply. But to be honest, I have not opened up a newer PSU that has a lead coming out to connect to the mobo's PWR_FAN connector, and those have to have some lead from the fan that provides the fan speed on the yellow line. So, if you have a PWR_FAN line out to the mobo connector, you'll have to look inside the PSU to find out.
However, you can just proceed without worrying a lot. Buy a 80 mm fan with 3 wires. The standard wires for mobo and case cooling fans are black = 0 vdc (ground), red = +12 vdc supply and yellow = fan speed output signal. When you get inside your PSU if it has those 3 wires, just hook 'em up. If the old fan has no yellow wire, just do not connect the new fan's yellow to anything.