How to identify remote keyless entry (RKE) frequency?
Most devices that work in the 315 MHz and 433 MHz bands use a transmitter that transmits at a precise frequency (there are certain channels that can be used, see LDP433) but the receivers are not that precise as they don't need to be (and that makes them cheaper).

To distinguish one transmitter from the other (to prevent your garage door being opened by any other transmitter at the same frequency), the identification of the receiver is not the frequency but in the code (a number) that is sent. The transmitters transmit a certain sequence using OOK modulation and that code must then be recognized by (a microcontroller in) the receiver.

What you could do is simply buy two receivers, one for 315 MHz and one for 433 MHz. In combination with an microcontroller and some software (using an Arduino comes to mind as there are ready made libraries for use with keyfobs etc.) you can easily retrieve the data that was transmitted.

If you only need to know if the device is 315 MHz or 433 MHz then you might only need the two receivers and for example an LED at the output.

In addition to 315 MHz and 433 MHz, some devices use 866 MHz, or 915 MHz, if you can find a receiver for that what I say above also applies. But I think 315 MHz and 433 MHz are the most commonly used. 833 MHz is somewhat less common.