If you want to check the riser efficiency .
Plug one GPU into the motherboard 16x and compare the power consumption with 6 GPUs on risers VS 5 GPUs on riser and one GPU on Mobo .
If you want to check the Power draw from the riser by the GPU then the easiest way is to use Dual PSU setup with the 2nd PSU only powering the risers .
Have you tried it?
But still, we need the different object, a few characters of cards. With one card only on test seems less accurate result. A different card may draw different power from PCIe slot or Risers.