Search for a frequency inside a wav file

I'm an absolute begginer of audio processing and I work with WPF in visual studio via C# and Naudio library.

I record a .wav file that contains Beeps in 0.5sec duration and frequency of 10KHz 10.5Khz (something similar to On-Off keying methode).No gaps between the beeps. My start delimiter is 9Khz. I start recording this series of beeps and stop it manualy. Is there any efficient way to search in the wav file for the start delimiter in order to decode the data?

Maybe using FFT or something like that? I thaught of "running" on the wav file and do some FFT on parts of the buffer but it sound like waste of time resources.

Thanks