you'll get used to seeing it: if you regularly jump 2'6, you'll start to see the difference between that and other heights, for example
typically, standard holes are 3in apart and start at 6in off the ground. so the first hole is 9inches, 2nd hole 12 inches. if you have a photo of the fence or can see the fence after your ride, you can determine it this way.
Its something that you just need to practice but a trick that friends and I use for measuring when we are on the ground is learning what height your hip falls at. You can than stand next to a jump and then estimate.
It's really just exspirence and comparing it to other things around that you know the height of