You can tell parts of your body from one another because each is faithfully mapped in the neural tissue in your brain. Your brain maintains a complete map of your body with patches devoted to each finger,hand, toe,etc. Your brain also maps the space around your body when you enter it using tools. For example when you take hold of a long stick and tap it against the ground as far as your brain is concerned your hand now extends to the tip of that stick. Blind people use this ability to feel their way down the street.
Your brain uses these maps to construct your body schema (this is the felt experience of your body).This schema is updated constantly by the flow of sensation from your skin ,joints, muscles, and viscera. The sense of inhabiting a body embedded within a larger world stems in large part from this mental construct. Anything which participates in the conscious movement of our bodies is added to to the model of ourselves and becomes part of the schemata. Any object used regularly can become part of your schema.Thia explains why people get so upset when someone hits their car, it is as if they themselves have been hit.
When we practice tai chi we are working on body schema awareness. We purposefully attend to many core elements of our own schema and also explore how our schema can enter our partner and (with practice) give us much information about their body too.