Chris Domeney, head of youth offending services in Southwark, said: "Poor attendance is one sign, but it's particularly internal truancy when they arrive at school and register but do not attend lessons," he said. "That can mean their role is to do business."
Mr Domeney said other signs included meeting older teenagers at the school gates and getting involved in playground fights.
He is keen to distinguish between casual groups of pupils who hang around together because they are bored, and those in proper gangs.
Southwark's youth offending team only define a group of youths as a gang if it has more than three members, a name, an established turf and is involved in criminal activity.
Southwark has taken a lead in tackling gang-related issues in London, though it insists its problems are no worse than in any other of the capital's boroughs or Britain's cities.
Projects include "anti-violence councils" in secondary schools and a 14-week gang intervention programme for pupils in Years 10 and 11, warning of the dangers of weapon use.
It also runs the "Wasted programme" in which pupils are taken into a hospital's accident and emergency ward to consider the life-threatening consequence of carrying a knife.
"A lot of the young people joining gangs feel dispossessed, they are failing in education and think by getting in a gang they can get instant street credibility," Mr Domeney said.
"We show them that the respect they receive isn't because of who they are or what they do - it's just fear. And that's not a valid reason to be respected.
"The reality is that gangsters often live in fear themselves. All they get is some ill-gotten money which most of them never get to spend because the price they pay is their liberty, or their lives."