I want to know two things.
Can we send a signal to a nerve using external source like electricity?
Can we differentiate signals sent from receptors like pain receptors, pressure receptors etc.
Yes. One of many examples would be transcranial magnetic stimulation (TMS).
Often, yes. I work with Drosophila (fruit flies), and we can differentiate between the electrical activity of sensory neurons, even those that innervate the same structure (e.g. sensillum) based on the amplitudes in the recorded electrophysiological signal. Neurons usually fire within a certain narrow range in terms of amplitude and that's one of many ways to differentiate between different neurons, or determine how many different neurons are found in the region where you are poking your recording electrode. Of course, if two neurons fire at the same amplitude, you would not be able to tell them apart using this electrophysiological method.