The same way we know who is going to commit what crime now. There are no guaranteed signs, just clues and maybe even historical patterns of behaviour. So ultimately, you don't and can't know for certain.
But you do assume a portion of the population (currently estimated at 15-20%) may have medical problems that affect their daily life and provide enough accessible public welfare systems that try to help people experiencing those problems, and you also foster a culture where getting help isn't a declaration that you're broken or weak. You also keep an eye out for your friends and family who might have been behaving unusually or... you know, radicalising. Normal collaborative society stuff.
None of us know when we might experience illness of any variety, including ones that affect our brains. Biology and chemistry often do weird shit, organic creatures have significant construction variation.
Society and community is a large part of how humans have prevented unexpected problems from killing humans unnecessarily. It is also how we should be preventing people from exploiting others.