The short answer is that it’s difficult to know. If the experience directs them towards God then it may have been a true holy ecstasy, but it if directs them away from God then it was false.
Could you give a little more information about what you’re questioning or looking for?
DISCLAIMER: Catholic Answers has turned over the archive to Catholic-Questions.org and no longer owns, manages, or moderates the forums. For additional apologetics resources please visit www.catholic.com.