Might be easy to test.
But it would take time. Once exposed to the air, ice like this should ablate faster, and by taking detailed photos over several days, you could probably see a difference in the appearance, particularly if you compare the borderlines of the parts still covered by dirt and exposed parts.
Of course, soon they'll be digging holes, and find out anyway, but such observations might still be useful in seeing how exposed ice behaves in the Mars atmosphere.