Hi there,

if I'm understanding you correctly, you need to determine
the frequency of a given sound, then check if that frequency
is  correct.

To extract the frequency from a given sound, you can
use a "Fast Fourier Transform", usually referred to
as FFT. Depending on the source of the sound you may
need additional filters/checks/etc. to be sure to extract the
main frequency and not some harmonics.

The FFT algorithm isn't very complicated to implement
and  you can find some Java implementation using Google.

Hope it helps,