What are the white circles just before the runway holding point? [closed]

These white circle were also marked with some range and bearing data. That means if your aircraft is in that circle, your vor and dme should read those marked values which in turn help the pilot to check serviceability/accuracy of vor dme. It’s manual version of test Vor to check accuracy of aircraft equipment.

If they look like this one at KSJC [https://www.google.com/maps/@37.356458,-121.9258751,158m/data=!3m1!1e3]

then it is a certified VOR receiver checkpoint. These will be listed in the chart supplement (A/FD).

San Jose (Norman Y. Mineta San Jose Intl) 114.1/SJC G 125 1.4 On runup area Twy W and D.

O Standards for Airport Markings should have information on all markings for US airports. It has this information on the design and dimensions of VOR checkpoints.