the vibrations are intense like chainsaws or electrocution but without the pain. You can't mistake them.
Stookie is right, you will know when they come, especially when your heart chakra begins to pulse furiously. The vibrations never just come to me. After I get into a full trance, I start an exit method (which varies), and continue to do it until I perceive the "vibrations." If you cannot remove your arm, its probably because your energy body is not fully developed. Once you feel vibration, thats probably a good time to try and separate you arm.
When you think your deep enough, double your relaxation. Keep going deeper and deeper...
What is the best level of trance for an Obe?
Cavernstoy