Surely it’s obvious, isn’t it? A sudden transformation from ordinary, everyday young person into a zealot raging about jihad and praising the savage killers of Syria or Libya. That’s the way it works, isn’t it?
If only such easy assumptions were true.
No, the indicators can be much more subtle and nuanced than that, and all the harder to pick up as a consequence.
Just how hard it is struck home with me about six months ago when I had to complete training in the Prevent programme, the Government’s anti-terrorism strategy aimed at identifying people at risk of being radicalised so the process can be nipped in the bud.
Some work in further education made the training a requirement, as it has been for half-a-million people in the public sector. I suspect, like me, most of them had no idea how difficult it can be to spot the telltale signs that can lead to the horror unleashed in Manchester.
That opened my eyes and unsettled me in equal measure. And yet, at the end of the day it took to complete the online training, I was left with a sense of hope because Prevent is one of our most powerful weapons in the fight against terrorism.
The hope was reinforced by a tutor whom I know at a Yorkshire college whose Prevent training led him to seek advice about a student who displayed an abrupt personality change, turning from friendly and easy-going into hostile and confrontational.
An intervention followed, with counselling, that revealed a troubled young man in his late teens, uncertain of his place in the world. He had not yet embarked on the route to radicalisation, but had looked at Islamist propaganda on the internet.
There was a risk that seeds of hatred would be sown in him. With help, and the support of his family – who were horrified to discover the direction in which their son might be drifting – he was steered away from that and is on the way back to being a happy student doing well in his academic studies.
A week on from Manchester, there has been much soul-searching about how to identify those at risk of radicalisation long before attacks are even planned, let alone carried out.
Amid that, there has been criticism of Prevent, much of it rooted in nervousness about offending Muslim communities.
No anti-terror programme is infallible. That Abedi slipped through the net of intelligence is proof of that, but Prevent increases the chances of spotting others who might be brainwashed into following his example.
Criticism is unfair and often uninformed. This is a rigorous, sensible and thought-provoking programme that steers clear of cliché or obvious answers and digs down into the psyche of those in danger of being led down a path of violence and murder.
It keeps the terrorist threat in perspective, and offers invaluable guidance in recognising changes in behaviour that might indicate that a vulnerable personality is being psychologically manipulated.
There may be a withdrawal from groups of friends at school or college, a growing insularity and reluctance to participate in activities they previously enjoyed.
There might be a falling off in the quality of homework, or a sudden dip in attendance.
Taken individually, such signs may not seem especially significant. But if they start to form part of a pattern, then there may be cause for concern.
Inevitably, Prevent is hedged about with ifs, buts and maybes. It is at pains to emphasise that when dealing with young people, changes in behaviour can indicate many things, whether it be difficulties at home, becoming stressed over studies, or relationship problems.
It is also scrupulously fair in not demonising Muslims, laying stress on the fact that a new interest in religious observance should not be taken as anything other than the embracing of faith.
Prevent’s value lies in making people think the unthinkable, and recognising the importance of being prepared to contact a helpline to report concerns.
It worked for at least one young man I know about. It will work for even more if the training programme is expanded, as it should be. Catching radicalisation early makes it much easier to counter than tackling it when it has hardened into a poisonous and murderous hatred.
There is no reason for Muslim communities to have reservations about Prevent. It is a programme that is explicitly even-handed to them, and seeks only to enlist their help in stopping attacks that injure every section of our society, regardless of race or creed.
The next Government should stress that, reaching out to Muslims to enlist their help and support in strengthening Prevent is one of the best shields we have against any repeat of the evil of last week.