Hehe.. Dave does have some valid points but I'm with Rob here, tough I'd settle for it being a 400mW zener. I guess you can chalk it up to experience/recognition..
As Resqueline says my conclusion was drawn by recognition and intuitian - i believed the OP wanted a quik fix as opposed to an explanation.
Oh and davenn the number written on a diode (as far as i have ever known) represents the breakdown voltage of that diode which for a 24V power supply 27 volts seems quite approproiate.