> Wikipedia article on FSK
https://en.m.wikipedia.org/wiki/Frequency-shift_keying
> The demodulation of a binary FSK signal can be done using the Goertzel algorithm very efficiently, even on low-power microcontrollers.
The source links to the TI page for the MSP430, named because it originally sold for $4.30. While the original link is dead, an internet search reveals many side and college projects using this technique.