In an ungrounded system the Line-Ground voltage readings are dependent upon the capacitive coupling of each phase conductor to ground. In a perfectly balance system the (3) L-G voltages should all be about 277V on a nominal 480V. If the loads are not balanced then the L-G voltages will move away from the ideal 277V. L-G current (i.e. from a ground fault) will also cause a shift. The extreme case being L-G = 0V on a solidly grounded phase.
Part of the problem you face, is not knowing what your base L-G voltages have been.
Ground detectors are not usually much fancier than the 3-light system, as Greg mentioned. I prefer to get a detector that includes a horn or at least alarm contacts.
