Since a lot of people have been wondering about sensing Vizard and Arrancar reiatsu, I will clear this up now.
ONLY Vizards and Arrancar can sense the duel reiatsu within each other. Arrancar can sense it within Vizards and Vizards within Arrancar, as well, of course, within their own race.
People outside of those two races will not be able to sense both. They will only sense the more dominate reiatsu (Shinigami reiatsu within Vizards, and Hollow within Arrancar) Upon release of an Arrancar's Zanpakuto or the donning of a Vizard Mask, the other more dormant reiatsu will be more easily noticed.
If none of those actions are done, then you will only sense something off with their reiatsu. Describe it how you will, but the bottom line is that you will just sense something is not quiet right with their energy.