Well, you know QFE as being zero feet when your aircraft is on the ground. Most if not all airfields have their elevation datum shown in the AIP or local web pages. At 30 feet per hecto-Pascal, you work out how many 30 foot (increments) there are in the airfield altitude. This gives the number of hPa which you then wind on to the altimeter. Your pressure sub-scale should now show the airfield elevation. Some airfields helpfully give a hPa difference as well as elevation, an example is here, not mathematically perfect but good enough for our use:
Welshpool