Blair says here how he deduced which posts were by Jarl: 

1) he said he mentioned to me that he sent a posting to the aeon-flux group about trevor being the one screwed around by aeon's impish and destructive acts.  demiurge99 wrote an initial posting to aeon-flux that said exactly this.

2) demiurge was one of his favourite aeon images/episodes after the characters of trevor and aeon.

3) the timeframe of posting was about right.  it also didnt continue beyond march 1999 (consistent with him not talking about any recent postings he might've done after march) 

4) the other groups that demiurge99 posted to were also exactly what jarl would find interesting (alt.teens.intelligent, alt.lifestyle.freethinkers, alt.tv.aeon-flux, and alt.tv.mtv-sucks).  The mtv sucks one is a detailed account of most things on MTV, broken down into what's quality and what's not. terms he'd use.

5) his use of his commonly-used "attack" word in the aeon-flux postings. I.e. "[he states his opinion and then writes] go ahead, attack me."

6) his mischievous replies to doofuses replying to his postings.

7) almost all other postings to the group couldn't possibly be him (many are AOL users, which he wasnt a user of, for instance; other people have continued posting into the current day;  etc.).  This also pointed to the person behind demiurge99.

-Sherlock Homey.