Rule: Defamation requries: (1) untrue statement (2) published to a third party (3) causing actual damage.
However, defamation per se does not require actual damage, when someone defame about victim's sexual behavior, professional conduct and criminial history. http://www.dancingwithlawyers.com/freeinfo/libel-slander-per-se.shtml
However, truth is a perfect defense in defamation. As long as what you said is true, the other party cannot win.
Procedure: The other party can find you via this procedure:
http://www.emedialaw.com/how-to-identify-the-anonymous-online-defamer/
Your situation:
1. The other party can sue, if he actually has a license. If he didn't have a license he would not win.
2. It is untrue statement published to a third party. The question is whether the statement is defamation per se, or caused actual damage. You need to tell me more about 2. What is 在網上拉燃油group 的ID and why do you care?
