How do you check the loop current zero (4mA) and span (20mA) of a HART field instrument?

See HCF_SPEC-151 Common Practice Command Specification, 6.3.1 Loop Current Trim Procedure.

Commands needed to perform two point calibration of the field device's Loop Current value:

• Command 40 Enter/Exit Fixed Current Mode
• Command 45 Trim Loop Current Zero
• Command 46 Trim Loop Current Gain