If it helps, but they are normally a very large sized building next to or very close to the side of the motorway. Adjacent to which is normally a very large car park with some petrol pumps sited towards the exit point.
As a rule there are normally feckin' big blue signs, again by the side of the carriageway, that inform road users well in advance when to expect to see the afore mentioned areas.
Hope this helps?
BTBR