Found an answer to my own question:
So how, exactly, do we spot a suicide bomber?
By Charlotte Edwardes
Last Updated: 1:00am BST25/07/2005
He's a young man, sweating profusely, looking around furtively, carrying a rucksack or wearing bulky clothing. He might be from an ethnic minority - but not necessarily. This, according to security specialists, is the most likely profile of a suicide bomber.
Experts contacted by The Sunday Telegraph say that a potential bomber may lick his lips, appear nervous and have shaved all his body. He may be wearing clothing inappropriate for the weather and be thrusting his hands into his pockets. If he is carrying a bag, he may be checking it. He may also wear a baseball cap or other headwear that would obscure his face from closed circuit television.
These descriptions were given in response to questions about how the public should recognise a suicide bomber while travelling on public transport. But while officials and experts agreed on basic details on the appearance of suicide bombers, most disagreed on how to respond if the public found one. "Speak up and advise fellow passengers," the British Transport Police advised. "If you see someone doing something very dramatic, the train is moving and you can't consult staff, then tell your fellow passengers, speak up."
However, Will Geddes, the managing director of the International Corporate Protection Group, which provides security advice, said: "Do not, in any circumstances, speak out. Your actions have to be responsible. If you shout, 'It's a bomb', you will panic him into setting it off."
Security specialists and emergency services advised that the public call 999 if possible. Scotland Yard, Transport for London and British Transport Police also urged people to alert staff on the Tube and buses if they saw something suspicious.
Asked what makes a person suspicious, a British Transport Police employee said: "I'm not sure. Someone fiddling with a rucksack?" His colleague added: "Generally, anyone suspicious will be with a suspicious bag, package or leave a suspicious unattended item. It could be anyone - not just Asian. One of the bombers was Afro-Caribbean."
He said: "If you see something, wait until you arrive at the next station then tell staff. Obviously if something more dramatic happens, you can hit the emergency button. But from the point of view of your safety, and that of fellow passengers, it's better to wait until you get to the next station. If you have a pen and notepad, note what you see."
Security analysts gave bolder advice. Robert Ayers, a former US intelligence officer and adviser at the Chatham House think tank, said: "You look for a combination of factors - are they carrying a package, are they nervous, sweating, licking their lips a lot and checking the package?
"If it is a radical Muslim, he may have removed all of his visible body hair. Look for a person who looks uncomfortable, anxious and out of place. They would most likely be from a particular ethnic background and aged between 20 and 35.
"The first thing you should do is to get away from them and get the police. If you can't escape or call 999 because you are on the Tube, then do not approach them. You'll panic them into setting it off prematurely."
Mr Ayers admitted that he would not alert fellow passengers. "If I thought someone had a bomb, I would go to another car. If I raise the alarm, he's going to set it off. If you see him move to detonate it, well you can only hope your life insurance is paid up."
Asked whether the public should look for a female suicide bomber, Mr Ayers said: "It's more than likely they would be male."
Mr Geddes disagreed. "There is an increasing number of female bombers," he said, "so I wouldn't rule it out." His general advice was to remain highly vigilant. "There is no precise science to spotting a suicide bomber but you can assemble a picture. Be aware of who is in the carriage with you.
"Is anyone carrying a package? Where do they put it? Do they leave without it? Don't be afraid of saying 'Whose bag is this?' If no one claims it, act. Be very careful not to cause panic. Wait until the train gets to the station, press the emergency button and calmly suggest everyone leaves. Say, 'I don't know who this bag belongs to. Let's get off the carriage and alert the authorities'. Don't do it in the tunnel. If the driver stops the train in the tunnel you're trapped. If it is something bad, there's no quick escape."
He said there were various methods to identify someone was wearing a device. "Are their clothes inappropriate for the weather, such as a heavy raincoat or fleece in hot weather? Another give-away is their physical shape. Generally, average-sized legs mean average-sized body. If they are much more bulky than their legs, neck or face suggest, that might be suspicious. Is the clothing disguising something?"
Mr Geddes said a bomber's mental state would present clues. "People who are about to undertake something high-risk will be perspiring. Look at their hands - where are they? In their pockets? Next thing: eyes. Are they furtive? Are they looking at the exit? Are they looking around the carriage? Are they making an obvious attempt not to make eye contact? Be conscious of the whole picture."
His final warning was that, after the past two attacks, a potential bomber might try to disguise his appearance. "Any terrorist group that has been watching the news will say, 'These guys are going to get caught in hours. They've exposed themselves to CCTV. They may make more effort to disguise themselves next time."