Feeling "at one" with the universe
This one is pretty easy, at least. Since when you enter a transcendent state, you are outside of space and time, you are also, in space time terms, everywhere and at all times. You have access to the transcendent states of all other things at all times (including your own), and as such a feeling of "oneness" with all things is very likely.
Coming at it the other way is tougher though. Does a "feeling of oneness" truly indicate a spiritual state? Or just one of complacent egoism? I suppose one clue to look for if you're not sure is the out of time and out of place aspect of a truly spiritual experience. Do you seem to hover or vibrate in and out of the real world while this happens? Does the world seem diaphanous and one-dimensional? Do your ordinary senses seem out of whack, delivering a shimmery, inconsistent flow of information to your sense of mind?
If so, you might be in a transcendent state... or you could just be going through a psychotic episode. It may be difficult to tell the difference, especially for those to whom you try to explain what happened to you - and even more so if you have a history of consuming mind-altering substances!
© Huw Powell