A sigil is the easiest way to this, but I don't know if you guys even use sigils. An idol works too. Get an entity that specializes in refining psychic senses. Good examples include Apollo, the demon Azazel, the archangel Gabriel, and the planetary entity Mepsitahl(found in the Art of Summoning Spirits by Konstantinos).
Open the sigil, or connect to the entity, and ask a question. The first sentence that comes into your mind, the first thoughts you have, write them down. Don't try to stop it, or edit it, just keep channeling. Repeat this twelve times, with NO self-doubt, and read back. You will clearly see that those words are not your words, and that you are indeed communicating with something else.
Repeat the process, over and over again, until you're better and better at it.