Put the Opamp in Open loop configuration and tie one of the inputs to bias level of VDD/2. To the inverting input, give another DC source and start sweeping the voltage till the ouput is at VDD/2. The input voltage will give input-offset/Systematic Offset Voltage. Make sure that you do very small increaments.
Another way to measure the systematic offset is to tie both the inputs to VDD/2 or some other comfortable bias level. Measure the output voltage. Now, input referred offset would be [Vo- (VDD/2)]/ DC gain of the amplifier
I hope that the point is clear