Since we can't actually "see" high energy photons, how do astrophysicists know what they know about high energy sources? There are two basic tools: spectra and lightcurves.
Photon detectors can keep track of what energy photon comes in and at what time that photon came in. Using these two pieces of information, astrophysicists make spectra and lightcurves. This information can also be used to make images, if the detector has the capability to determine position as well.