|The best time would be a few hours after the sun has set. Preferably on a clear night, stars, no wind. The heat from the sun during the day will heat the insulation under the surface; any water that is trapped in the insulation will show hot at night while the dry surfaces will cool faster to sky. Water has a higher thermal capacitance so it will hold the heat longer. Be careful when you see a hot spot, it could be caused from other factors such as heat from an exhaust vent or a piece of metal flashing. When in doubt verify with an EIFS moisture meter. Hope this helps some.
Building Science Thermographer