You will often be able to distinguish arteries and veins, especally when they run together, as they usually do. Arteries have thicker walls and tend to have narrower lumens. They have to constrict and dilate to control how much blood flows where, and they must bear the powerful force generated by the heart. Because of the large amount of muscle in their walls, they are usually round when cut in cross-section.
Veins have thinner walls and tend to have wider lumens.
They have to store blood, and need muscle only to push the blood back to the heart. Because the walls are collapsable, they may change shape depending on the surrounding tissue conditions. This is a vascular group in the pancreas. The endothelial nuclei in these arteries are easy to see.