Generally, when you talk about meditation on Om, you do it only when you come to Mandukya Karita. Otherwise, remember, when you say `I am meditating on Om’, `Om’ stands for `Ishvara’. Therefore, truly speaking, I am meditating on Ishvara. I am using a mantra to focus on Ishvara. Om also stands for the Reality. Therefore, meditation on Om can also mean `nidhidhyasanam’ or contemplation on the non-conceptual, on Reality, on the pure awareness, Brahman that we have been talking about.
If I am contemplating on Ishvara and if I find Ishvara becoming more and more real in my life to the extent that I start recognizing everything as Ishvara, if I feel a sense of divinity, and if I function naturally as a karma yogi, then I am doing it right. If I am using it to contemplate on Reality, the more the Reality is clear to me, the more certain I can be that I’m doing it right. There are no other signs for this unlike energy meditations etc; in the course of which there may be some physical signs e.g. I may feel some movement of energy in some part of the body or the other. Here, as this questioning is concerning the Reality of Ishvara, such signs will not be there.