Throughout the history of mankind, religious leaders - Moses, Muhammad, Jesus, Buddha, and many others, have recorded experiences which they considered ‘religious experiences’.
But what is a religious experience? What makes an experience religious?
Since here it is hard to put a precise definition on what a religion is, it is even more difficult to precisely define a religious experience.
Instead, it is much more productive here to use Wittgenstein's concept of family resemblance, that is defining a category by the set of prototypes and examples of what a member of that category looks like, but without any clear set of features separating the member from the non members.
In fact we can take the description of family resemblance from the SEP article L.W, and modify it, by replacing "language game" with "religious experience" to obtain a good description of what a religious experience is:
Still, just as we cannot give a final, essential definition of ‘religion’, so we cannot find “what is common to all these experiences and what makes them into religious experience”.
It is here that Wittgenstein's rejection of general explanations, and definitions based on sufficient and necessary conditions, is best pronounced. Instead of these symptoms of the philosopher's “craving for generality”, he points to ‘family resemblance’ as the more suitable analogy for the means of connecting particular uses of the term religious experience.
So for your question, it would be useful to list a number of religious experiences: Enlightenment after meditation, revelation by supernatural means, radical changes in worldview and spiritual outlook, etc... and then compare a new instance (i.e. an experience that we want to determine is religious or not) against that list and see if it bears any resemblance to its items.
Utter certainty that what one is experiencing is the TRUTH is probably a necessary characteristic of such experiences. (William James says that this experience has value for the person experiencing it but not necessarily for anyone else. However, I suppose people are influenced by such persons and a cult and perhaps even a new religion is born.)