No clues on route sheet.
Really? You must have a different route sheet to me. According to my version, the Bough Beech control is between 22.4 and 24.0km, so that's a pretty small window. And the next instruction after the info is "Pass over the reservoir causeway" so it should soon become obvious if you've missed it.
The info at the top of Exedown should also be obvious - it's usually a number on an AUK-branded sign. Besides, at that point of the ride, you won't be riding alone. If you see other riders stopping to write something down, copy them.
If you start descending, you've definitely missed it. In which case you will have the dubious pleasure of having to climb Exedown twice.
d.