I'm going to assume this is a single phase motor.
So simple speed control CAN be done on CERTAIN TYPES of single phase AC induction motors, called Shaded Pole (SP) motors and as it so happens, that is a common type used on fans because in general, SP motors are not very powerful. With a Shaded Pole motor, you can simply reduce the voltage, say with a light dimmer and the motor produces less torque, then with less torque the air friction on the fan blades causes it to slow down, which in turn moves less air. Shaded Pole motors are inherently current limiting so in slowing them down, the motors WOULD overload, but can't because the current cannot increase so they just lose torque. If your motor is Shaded Pole, it should say so on the nameplate, or it should at least say "SP".
There are some types of fans that use what is called a Permanent Split Capacitor (PSC) motor, they too can be controlled by simple voltage reduction, but are at greater risk for long term damage if they over heat. Many overhead ceiling paddle fans are PSC type and can have their speed reduced with a dimmer because the case is designed to dissipate that extra heat. If you are not talking about a ceiling paddle fan, I would not attempt it.
Basically if your motor has a big "bump" on the side, like it is about to give birth, that's an indicator that it is not SP or PSC and you cannot change the speed.