With my scope behind the rifle, or with steady/mounted binos I look for a horizontal and clear defined light colored edge somewhere near the target, like the top or bottom of a target placard or the top/bottom white edges of the target itself, especially metal things. I find those edges tend to give away the mirage pretty well and the ripple gives you direction. With a spotter you can usually see more in the air itself, dep upon conditions as said above.
I know some who can dial up magnification on scopes and adjust their dope for the mirage effect on poa=/=poi, but i usually dial down to @ 10-12x to minimize the distortion on poa-poi through the mirage. I haven't gotten to play around with those shifts yet...