Dentist thinks i brush too hard. another said maybe i grind my teeth. not sure if its true but what signs could they have seen ? leads to gum recessn?

Tooth brush: Do not use medium to hard tooth brush for brushing, otherwise you would have gum recession. In this case the lesion is generalized (occur in many sites throughout your mouth). Grinding teeth or occlusion as a factor causing gum recession will produce a more localized effect, over the area where malocclusion occur.

Signs of grinding: They may have seen a flattening the biting surface of the back teeth, or incisal edges of the front teeth. The points or cusp tips would be rounded off if you are a grinder. Also if you are a grinder there could a cupping of the tooth at the gum line, this would be glass smooth to the touch. Another sign could be a stripping of the gum, either isolated or generalized throughout the mouth.