Toe dragging is a sign of hock lameness.
A stabbing motion with the hinds, or unwillingness to put them forward indicates a stifle issue.
Unwillingness, issues with things previously found easy by the horse, issues backing up, etc.. are also all going to be indicators of a horse who is uncomfortable in the hind end.
Lamenesses in the lower legs (ie pasterns, coffin joints and hooves), on any leg are easier to see as the horse will usually be reluctant to put any weight on the limb at all, and the horse will head bob.
The easiest way to spot a hind end injury is to watch the arc of the toes and the arc of the hocks - both should match and be clearly arcing with no jerking or dragging.
Horses are also highly affected by the trim job, even behind, and a bad trim, and more likely subsequent bad trims, can lame a horse behind.
Here is a horse showing good even "arc" behind
I would call that a baseline for a sound horse, any horse not tracking like that needs to see a vet and possibly a new farrier.