as an anesthesiologist, i always used the air in the syringe technique: gently ballot an airfilled, liquid lubricated, glass syringe attached to the Tuohy needle until there is a sudden, "give way" and the air injects (don't inject much air). i saw a "hanging drop" technique used by an attending once and he got a wet tap!
we never used XR. this was both for CESI and epidurals for labor pain/C sections.