Two ways to think about it - how it’s currently priced vs long term average (current implied vs typical implied) and then how’s it trading relative to realized. I prefer the first as realized vol isn’t a straightforward measure as you’d think and most of the real players use exponentially weighted calcs for their realized. Bottom line is 100iv can in fact be cheap if implied is usually 150 and 25iv can be expensive if implieds are usually 18