How do I find spectral purity?
Spectral purity is the stability of a signal.
Your picture is an FFT which shows a clear peak slightly below 10 MHz. However, it's not a pure 10 MHz. A pure spectrum would be an infinitely steep spike, visible only as a vertical line.
Practical you'll never get such a pure spike. You'll always see harmonics, non-linearities, phase noise, mixing products, etc., perhaps even all of them. Your spectral purity is similar to SNR in that it's the relative amount between 'wanted' and 'unwanted' signal.
Spectral purity is the inverse of energy resolution.
Energy resolution is the ratio of the full-width-at-half-maximum (FWHM) of the peak divided by the centroid position of the peak. For more information take a look at Brittanica#Spectroscopy-systems. FWHM is expressed as a ratio or a percentage. Smaller value = higher purity (so for spectral purity it would be higher value = higher purity).
Although I can't say for sure, your energy resolution seems to be lower than 2% (based on pictures at h3dgamma).