ALA inspection in their air is part of basic PPL training.
You should have an awareness of how far your aircraft travels with lots of flap hanging out (taking into account adjustment for estimated headwind) so you can mentally count one-thousand, two-thousand, three-thousand etc. and get a good approximation for the airfield length.
eg.
60 knots = (just under) 31 metres per second.
70 knots nil wind = 36 metres per second.
80 knots nil wind = (just over) 41 metres per second.
If you're doing 70 knots nil wind for 20 seconds over the ALA then the length is about 720 metres long.
Don't forget your fudge factors too.