With difficulty:
Could be a broken tile/slate where the broken part has slid out leaving the gap in tiles/slate below it for rain ingress usually a heavy leak. It could be soakers have gone porous these are under seals where the tiles/slates meet a chimney wall and are covered by an overlap apron closing in the rise up the stack wall (about 3") Some just have a cement fillet (bodge job) which has opened up. See in attic if you can spot a wet/damp stain on one of the rafters, this will give some indication, follow stain up the rafter and you should find the leak.
Bob UK, January 2014
