Measuring amplitude of a spectral component
A variation on @geometrikal's answer would be to use the Goertzel Algorithm.

$$
y[n] = x[n] + e^{+j\omega_0} y[n-1]
$$
where $x[n]$ is your input signal, $\omega_0$ is the frequency you know, and $y[n]$ is the (complex-valued) estimate of the discrete Fourier coefficient.