Where I work, we do it by saturated suction temp & actual pipe temp difference.
Presuming that saturated suction temperature is determined by measuring suction pressure and converting it to temperature with a P-T chart? That is the preferred method.

Note that attempting to set the TEV to control a low superheat, say below 4°F, can be tricky. The problem, of course, is the accuracy of one's gauges and temperature probe. The Prof recalls a technician who was amazed that he could be the TEV to control an absolutely steady 1.7°F superheat. The Prof, of course, suggested that he adjust the TEV in somewhat, and get his intruments better calibrated if he plans to continue having the TEV control low superheats