A cracked or bowed panel usually shows first as daylight at the seam between sections, or a visible gap at the bottom when the door's down. Rust starts as a rust-colored stain bleeding through the paint near the bottom edge, then works into a soft spot you can push a thumb into. If one section looks like it's sitting slightly forward or back from the others when the door's closed, that's often a bent panel pulling against the track rather than the track itself being off. A dent that's purely cosmetic - no crack, no rust, door still runs smooth - is sometimes not worth the panel swap at all; we'll tell you straight if that's the case rather than selling a repair a door doesn't need. Sagging across the whole door, as opposed to one section, usually points to the spring or the top panel's connection to the opener arm, not a panel problem, and we'll say so if that's what we find.