The Jargon File (version 4.4.7, 29 Dec 2003):

Troll-O-Meter
 n.

    Common Usenet jargon for a notional instrument used to measure the
    provocation level of a Usenet troll. ?Come on, everyone! If the above
    doesn't set off the Troll-O-Meter, we're going to have to get him to run
    around with a big blinking sign saying ?I am a troll, I?m only in it for
    the controversy and flames', and shooting random gobs of Jell-O(tm) at us
    before the point is proven.? Mentions of the Troll-O-Meter are often
    accompanied by an ASCII picture of an arrow pointing at a numeric scale.
    Compare bogometer, Indent-o-Meter.