All of inverter has an input filter capacitor (Typically for Desktop monitor and LCD TV Inverter).
The positive pin is connecting to Input voltage supply and the negative pin is connector power ground
and metal chassis (Photos shown below)
Locate a capacitor.
Set your multimeter on DC Voltage Range
Connect black probe on power ground (or Any part of metal chassis)
Connect Red probe to Positive pin on the capacitor
Turn on LCD Monitor and the Input voltage are display on the multimeter