The best way is to listen for short-wave drop outs caused by solar flares. When solar flares erupt, the D-layer just below the daytime ionosphere becomes highly ionized and acts to absorb short-wave transmissions on long- path bounces. You can listen to distant stations, and when a flare happens, within 10 minutes the distant station 'drops-out' and becomes inaudible for several hours until the excess photoionization in the D-layer wanes to its normal level. This is a chemical process that takes time to proceed because of the very low densities of the gases at these altitudes. I am not a 'short wave person' but my understanding is that it affects all bands in much the same way from 1 megahertz upwards to the ionosphere cutoff near 10 megahertz where it starts to become transparent to high frequency radio waves. Solar storms also produce several different classes of bursts of radio emission, but you need a radio telescope set-up with a very sensitive receiver and a 'dish' to detect them.
All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the
NASA IMAGE/POETRY project.