DC offset is measured at the output with the pot turned all the way down. With a DMM (digital multimeter) set to DC volts, you measure from L (tip) to ground (sleeve) and R (ring) to ground (sleeve). These should ideally be <1 millivolt (0.001 Vdc), but anything under 5-10mV should be fine. Easiest way to do it is to use a blank 1/4" phone plug and do your measurements across the contacts; avoids having to open the unit.
Excessive DC offset could trash your IEMs. I'm not pointing fingers at the manufacturer or anything, but since this is a DC coupled amp, there are no coupling caps to protect you from this.
One other thing you could try is unplug the amp from your source, turn the volume all the way down, and then listen. If you still hear the noise, then it is the amp. If not, blame your source.