You can spot them because they often carry two sheets of sticky labels. One is blank, to cover up spurious apostrophes that they find, such as at the greengrocer's stall at the market. The other contains apostrophes which are inserted where they are missing, often at banks: Barclay's, Lloyd's TSB, and supermarkets: Tesco's, Morrison's.
However the world of guerilla pedants is a divided one. Although they all share the same standard when it comes to English as she is spoke and written, they lack a similar benchmark when it comes to communities. The Pedant's Society was torn apart (soon after their triumphant Millenium Party held on the 31 December 2000) by argument and a separate Pedants' Society was formed (both are correct).
The guerilla pedants, although sharing a common need to put the written word to rights, are not united in the tactics. There are those who think that if people only ever see correct English they will absorb it and only produce it themselves. So they carefully print the word "fewer" in the correct font and colour and stick it on the sign at the supermarket that says "10 items or less". Others think that people should know just how bad it can get, and cross out "less" and scrawl "fewer" in black marker, so that you know what's right.
Photo's by neonbubble and Will McGree.