For 27MHz RC toys, the specific frequency is usually 27.145 in the US. For 49MHz RC toys, 49.860 is common. However, radio controlled toys could use other frequencies. They may also have switches on both the transmitter and the vehicle which allow the user to select from up to 6 different channels within a certain frequency range. Both the vehicle and the transmitter must be using the exact same frequency in order to work properly.
If you have a couple of identical transmitters and aren't sure what frequency each one is, you can either try operating each of them with different frequency vehicles (easiest, as long as the vehicles are operational) or open up the transmitter and look at the frequency etched on the crystal.
I hope you've enjoyed this mini-tour inside a radio controlled toy transmitter. You may also enjoy looking inside a typical radio controlled toy truck.