Well here are some signs of abuse i've witnessed.
Aversion to human contact
Yelping at being touched (heavy abuse)
Missing patches of fur (signs of repeated beatings to the same area, usually the head)
urinating/defecating at loud noises or yelling
fur is brittle and skin is very sensitive or red (malnourishment)
mange (infestation of mites, usually a sign of a lowered immune system which is linked with stress in dogs can also come from malnourishment)
Shittishness, anxiety, flinchy, barks at certain people, hides from people of certain genders/races/heights/weights, hides from certain objects. My dog was abused with something like a broom when she was young so she paces, shakes, & cries when someone walks around with a mop/broom/stick.
Malnourishment, wounds, skittishness....