People like playas for looking for the traces of ancient water, and the life forms it may have contained. If a lake had fish in it, and the lake evaporated, the fish fossils might be at the bottom, for example. But I think it won't be so easy on mars, because if the lakes existed 3.8 billion years ago, the playa surfaces at that time would have been churned up by impacts since then, which might destroy or dilute the original materials. Also, many surfaces have been covered by dunes and other wind blown deposits since then. That's why it's a challenge.
Nobody has a good answer here.
There are debates about whether water lasted for longs times or short times. And finally, spectrometers confirmed identities of CO2 and H20 ices. When you see white frost appear on surfaces where the temp drops below 0 C, it is likely to be H20 frost. The same is true for clouds. The C02 frost and clouds form at much colder temperatures.