There are two kinds of powerful mage: The dedicated kind and the talented kind.

To spot the dedicated mage, look for intense and constant concentration, lots of trappings and books, a devout seriousness, and probably an older person. It takes a while to become a powerful mage this way. You can always believe a dedicated mage, but they talk like Obi-Wan Kenobi.

The talented mage is often younger. Look for amazing luck, impossibly fortunate "coincidences", and piercing eyes. Very often, a talented mage doesn't know that they're a mage, so beware of folks who can walk around raindrops and laugh at the "Occult". This talent often manifests in more ways than one, so look for another, corresponding talent to be present as well. Oh, and you can't always trust the talented mages; they didn't hone themselves to that condition, and they may be jaded with everyone else's lack of common sense, as well.

Happy hunting!