I assume that you mean a power inverter (DC to AC "step-up" converter).
You would need a means of sensing the current. You can use a shunt for this or a current transformer or a hall-effect sensor.
A hall-effect sensor may be best, but is likely to be very expensive.
You can use a shunt, but keep in mind, the voltage and power loss introduced.
To get an indication of the current, measure the voltage from the sensor (whichever you decide to use) and compare against a reference (voltage you will get at maximum load/current). Voltages may be low and amplification may or may not be needed.
Hope this helps.
Tahmid.