either refer to the sticker on it; if it doesn't have one, then use a multimeter. measure the (DC) resistance through one of the coils (they'll be the same). DC resistance - known as 'Re' - is always a little lower than the 'nominal impedence' of the VC. so if it's a dual 2ohm model, the Re should be ~1.8ohm. if it's dual 4ohm, then each VC will have an Re of ~3.6ohm.