How would you distinguish between a renal artery and renal vein in a dissected kidney?
By the thickness of the wall of the vessel.
Since we're talking about blood vessels of same caliber (same diameter), we can compare the thickness of the wall of the vessels to determine which one is which.
We know that arteries have thicker walls than veins, because they have to endure much higher pressures than veins. So, if you have a vein and a artery in front of you, and you know that these are renal blood vessels by looking at its emplacement, you just have to look at the thickness of the wall. The thicker wall is the artery, and the other one is the vein.