User blog comment:Miskos3/Fairy Tail Episode 259: Tartaros arc: 00:00, Review/@comment-24784846-20151114170157/@comment-4829046-20151116072635

As a curious example of the touch sense thing, Erza would not have felt any real impact when hitting Kyouka. It's like when your hand goes completely numb and you find it difficult to tell between a pillow and a brick by touch alone even if if you slap on them. So she might've just been hitting a wall or even a sheet of paper and all she could use to judge the effectiveness of the attack would be the amount of pain received, which for obvious reasons couldn't have been accurate. In other words, the only real way is, like others said, the extra sense that may or may not be biological.