As Pike is doused with water attempting to extinguish the fire, quite a bit of water strikes the chimney. The water should immediately steam and boil away. It doesn't, obviously revealing the fire to be fake.
It is raining at the end of the episode. As the screen fades to black (the "You have been watching" caption appears), for a second the rain is still seen, proving it is superimposed instead of real water actually falling.
At times the dialogue during the roof scene is almost drowned out by the sound of the CO2 used to create the chimney fire illusion: a chimney fire would be almost silent on the roof.
At the end (before the credits came) it is 'pouring with rain'. This is fake as although the screen goes blank, the rain film is still shown.
During the garden excerpt involving the verger, Warden Hodges and Gerald, the background scenery is quite clearly a painting. One can even see creases and small holes in the canvas.
As the chimney is extinguished.