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How COULD a grass mark on a dune become fossilized? (Just curious as to what the traditional geology explanation is...if there is one).
Salts wicked up to to the soil surface by evaporating dew could act as a curing agent to stabilize the fossil, and you can gently bury it by thin sheets of sand collapsing down the face of an advancing dune.
However, this is a rare formation that you aren't likely to ever see in the rock record. An more obvious and more common feature you should look for instead are rhizomorphs, which is the plant's root system preserved as a fossil. Since we seem to be very desert-oriented right now, another thing you could do is slice up the rock into thin sections and look for cubical grains of halite (the mineral equivalent of table salt) and gypsums and the other evaporites and for pseudmorphs after them (that is, other minerals that have gradually replaced the evaporites but retained their crystal and grain shapes).
I would look for mud cracks in between dune beds in addition to the rhizomorphs, and as was mentioned I would measure the angle of repose of the dunes and I would look for flat sheets of sand at the toes of preserved dunes that indicate collapse. I would also be on the lookout for fossil soil horizons (paleosols) where minerals have been leached or worked by plants and redeposited shortly below during perhaps wetter climate intervals.
As I mentioned before, the sandstone should be very mature, that is, nearly all quartz and very rounded. If they appear opaque from being battered in transport, that means that they are probably aeolian.
So yeah, there are things that tend to distinguish windblown (aeolian) deposits from under water deposition.
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