Typical Semi-Detached house  (side elevation)
Schematic arrangement for a semi-detached house with fireplaces in the 2 ground floor rooms only.
This terraced house has four chimneys 
each of the two bedrooms has a fireplace.
Notice how the bedroom chimneys are connected to the outer chimney pots on the stack. So the general rule is:-
1st pot = front bedroom
2nd pot = front downstairs
3rd pot = rear downstairs
4th pot = rear bedroom . . . .

usually, in about 99.9% of houses. There is always the odd house that proves the exception though!
So the thing to do if you are not quite sure which chimney pot is connected to which fireplace, is to light a smoke pellet and watch outside which chimney the smoke is coming through. Simple, but very effective. You wouldn't believe the number of times builders and others who should know better have got chimneys and fireplaces mixed up, sometimes with serious consequences.