Figure of merit for 'locality of power'
You could try calculating Shannon entropy of the spectrum. Normalize the Fourier transform $f(x)$ of your signal so that $\int_{-\infty}^\infty |f(x)|^2\, dx = 1$ and calculate Shannon entropy as $- \int_{-\infty}^\infty |f(x)|^2 \log |f(x)|^2\, dx$. You can clamp $|f(x)|^2 \log |f(x)|^2$ to zero for very small values of $f(x)$ if its logarithm blows up and ruins the calculations. Zero-padding before windowing &amp; FFT will give you more frequency resolution and a better approximation, especially if you have narrow peaks in the spectrum.

If you want something more standard, have a look at spectral flatness. But it seems even less stable numerically.