Left and right ulna
The right and left sides can be determined by having the large notch at the upper end of the ulna (trochlear notch) facing anteriorly and the short process at the lower end (ulnar styloid process) medial to the ulna.
You can also determine left and right by holding the radial notch, which is next to the trochlear notch and where the articular circumference of the radius fits, so that it is on the outside.