You can't judge a book by its cover, but you can judge a hybrid with one look at its face. Even your kindly mother would say so, provided she's a golf techie who understands the dynamic relationship between bulge, gear effect and center of gravity.
Irons have flat faces, but woods are rounded from heel to toe. This is called bulge, or the measured radius of an imaginary circle whose edge lines up with the clubface (a shorter bulge radius means more curvature). The purpose of bulge is to mitigate gear effect, or the tendency of toed shots to hook and heeled shots to slice. With bulge, an off-center hit will start in the opposite direction of the ball's spin so that the shot finishes at the target.
As always, hybrids are mutts of mixed genetics. The amount of face bulge hints at whether one will play more like a wood versus an iron.
"Generally, the farther back the center of gravity the stronger the gear effect," says Tim Reed of Adams. "The stronger the gear effect the more we round the face to counteract it."
Conversely, if a hybrid has a flatter face, chances are its center of gravity is positioned more forward, like in an iron.
"This is why a large bulge radius provides a more workable hybrid for an accomplished player," says Matt Gecewicz of Sonartec. "Deeper CGs and rounder bulges tend to be more high-handicap friendly."
"Better players usually like clubfaces that look square," says John Hoeflich, senior vice president of Nickent. "But most major brands have figured out how much bulge plays appropriate to their version of a hybrid."
Also worth considering is that flat faces might rebound slightly less than round ones, and so that might be a better choice for the player looking for consistent distance control instead of total distance. Of course, if you tend to hit it in the center of the face, you might be less concerned about off-center performance.
An easy way to estimate the bulge of any club: Take a credit card and see how much it rocks back and forth against the face.