Hi Manze, I think we all have our own process and way of connecting and sensing the various energies of spirit. I believe that there really is no unique method that is correct or wrong - how it works for you is right for you. That said, for me the connection is very much like being in a room of people. If you close your eyes, you can often still sense the different presences and energies. So even if I can't 'see' them (which I sometimes can), I feel them - and 'see' them psychically. I also differentiate sometimes differentiate by the type of energy I feel, for instance a very overwhelming high energy is often an ascended master or angelic spirit for me.
On an additional note, I find that the deceased often appear initially in their very human energy - as they were in their life. This seems to be more about identifying them for the sitter, as once they confirm that identify, I feel the energy shift to a higher vibration - their spirit or true self.
I know this is a big lengthy - but I thought it was a great question, and it got me going lol.