Why is the moon red tonight?

When the Moon is rising or setting, its light travels through much more air before reaching your eyes. The atmosphere scatters blue light more efficiently because it travels in shorter waves — the same reason the daytime sky is blue; the more air a ray crosses, the more blue gets scattered away along the path, leaving the rest looking redder. The same thing happens to the Sun at sunset, when its light has to cross more atmosphere to reach us. Airborne dust, wildfire smoke, and pollution strengthen the effect, since those particles also scatter blue light: during haze or after a large fire, the Moon can look orange even fairly high in the sky.

The second possibility is a total lunar eclipse, and it's easy to rule in or out, since it can only happen at full moon and is known years in advance. During a total eclipse the Moon enters Earth's shadow, and the only light reaching its surface is light that has grazed the edge of our atmosphere; air molecules scatter away most of the blue, and the remaining light reflects off the lunar surface with a red glow. At least two partial lunar eclipses happen every year, but total ones are rare.

The practical test is altitude and duration. Atmospheric reddening affects the whole disk equally and fades as the Moon climbs higher above the horizon. An eclipse, by contrast, advances like a bite of shadow moving across the disk over several hours and keeps the coppery tone even when the Moon is high. And if the Moon isn't full, an eclipse isn't possible at all — what you're seeing is atmosphere.