Yesterday's extraordinary story of the undercover policeman, Pc Mark Kennedy, who used a fake identity to infiltrate a group of climate change protesters, raised many questions – not least of which was, what on earth are the police doing financing this sort of undercover operation anyway, and how many agents have they got out there?

As Home Secretary Theresa May faces pressure to explain the police action in Parliament, and Scotland Yard is brought to task for spending a fortune - £250,000 per agent, apparently – on infiltrating climate protest groups, where does it stop? Has your yoga group been infiltrated? Is your book club safe?

(Note: For every 'he', feel free to substitute 'she': Pc Kennedy is now reported by the Guardian to have exposed a fellow police spy – an undercover woman police constable who suddenly vanished from the eco-protest scene in 2008 claiming she had fallen in love, and has not been seen since.)

HOW TO SPOT AN UNDERCOVER INFILTRATOR – THE TELLTALE SIGNS

• Did he turn up out of nowhere, and get on famously with members of the group within hours of joining?

• Does his wife/girlfriend, whom he talked about at the first few meetings, suddenly vanish from his conversation?

• Does he disappear for long periods, claiming he's visiting relatives or going to an east European pop festival you've never heard of before?

• Is he a bit of a puzzle: he's terribly keen on your campaign, but you cannot quite explain why he's involved?

• Does he - on the other hand – over-enthuse about your campaign, urging the group on to ever more daring escapades?

• Does he ever accuse other people in your group/movement of being agents (to divert attention)?

• Does he appear to be excessively helpful – offering to ferry people to protests, take them home afterwards?

• Does he have a car - or a van - which he offers to make available to the group for transport to and from meetings/demos?

• Does he volunteer for tedious tasks which, while doing the rest of you a favour, allow him access to data, such as membership lists?

• Does he appear to be oddly well-off, buying rounds in the pub, having access to surprisingly generous quantities of dope, but never giving any indication of where he works or how he earns a living? (Pc Kennedy was known as 'Flash' for his generosity.)

• Does he ask for a receipt at the bar when he thinks no one's looking?

• Does he urge the group to be violent or break the law but – oddly – not show up on the day for which the activity has been planned?

• Does he seem a little old/straight to be wearing his hair that long, and sporting an earring?

• Is he fiddling with his mobile when the rest of you have yours turned off for an important meeting?

• Are you suddenly aware that there is more dissent within the group than before, more disruption?



• Are things suddenly going pear-shaped when they always went pretty smoothly before?

Answer YES to three or more of these, and you could have an undercover cop in your midst.