Locate the anterior aspect of Ulna. A good landmark is the trochlear notch.
When viewed anteriorly, the radial notch point to the side the bone is from, i.e. left ulna will have the radial notch (see below) on the left side of the bone.
Proximal end of Ulna, focusing on the radial notch. (Picture taken with a 120/4 and Tube 55 on Aptus 65)