The only way to be sure is to have a waypoint at which to carry out a RA/Baro crosscheck. In most cases with flat ground or water that's not a problem. Where it IS a problem, why not redesign the lateral track so that you DO fly over a something flat. And if there is nothing then use overhead the airfield at 2000', effectively turning it back into a procedural approach.
But you still have to remember to do the crosscheck.