There's aligned marker sets that you can usually go off of, a couple of the strings are pulled from marker to marker across the spot I''m digging at. So you can test your compass with those. In fact when I started I was about a 1/2 degree off on my compass for an alignment and it bugged me enough to call my brother to look up the current declination for my location while I was still in the field. I couldn't figure why one marker was aligned slightly off the bearing, it seemed the correct placement was a few feet to the side of the marker with current compass setting, but when I readjusted it for the extra 1/2 degree declination it hit just right. I may have still had it set for a place I went camping at last summer.
On the slope distance you can calculate the horizontal distance with an inclinometer if you can't do a straight pull on a tape measure. I just have a right triangle calc phone app that does the calcs now from angle and distance input. There were some I couldn't pull anywhere near straight, so I didn't bother calculating them yet until I find where I stashed my suunto .
On some some sites when you measure and use a 33 inch vara, when it's a couple inches from 20, 30, 50 , and 75 varas on 5 of your measurements, well, then it's probably pretty solid. 137.5 feet = 50 varas, a pretty common one. 82.5 feet another @ 30 varas. Eleven foot increments are pretty common, 11 feet=4 varas. This is the first site that I have tried a 20.8 inch cubit, measuring from the middle of each marker. The varas could work if you used the back of some markers vs the middle. But there should be a uniform way to take the measurement. And it bugged me enough to try a 32 inch vara, and then an old royal cubit, until things worked out to a spot about the size of my hand. On many sites though , it will be a mix of feet and varas. Smaller newer placed markers in feet, older larger ones in varas. Again many of the markers here had a chipped out divot on top of them where the measurement was likely taken from, unless they had an obvious place to pull from like a pointed tip top.