I think if you are asking this question, then you haven't been on a salt lake. Most salt lakes in Australia have a thin layer of Salt. The exception is the Southern end of Lake Gairdner where they do the salt lake racing. There the salt is up to 1m thick.
If you search pprune, you'll find many references to aircraft that tried to land on Lake Eyre that either came out by chopper or a still there.
Salt does have a crust and below it can be clay the consistency of porridge. And don't be suckered in by the 4WD track wives tale. A), the salt lake can change condition quickly and B) I've pulled a 4WD out of Lake Torrens that followed what he thought was vehicle tracks, but was actually 2 dirt bike tracks side by side (you reckon they didn't do that on puropse?).
Hard salt has a distinctive shade of white, but if your'e not familiar with the different shades of white with different degrees of wetness (read softness) then you won't pick it.
I haven't seen the salt lakes in Nth SA properly white / dry / hard in a number of years.
So, unless you have Tundra tyres or a trusted mate to drive it first - don't do it.