I know when I have it because I feel sleepy, but even if by chance I could take a nap, it is not useful.
Once the nap is over I feel like before and moreover have difficulties to stand up and go back to the old activities.
That is why I never liked to take naps.
I found out that when I have low blood pressure the best is to keep myself busy because if I sit a little bit on a couch and close my eyes, I don't want to stand up anymore.