From a piece I just found. No link as otherwise the post would be delayed. See note at end for source.
I think my oleophobic layer has worn off, what should I do?
"First thing's first, if you are unsure, whether the coating has worn off, drip a single drop of water on the display. If the drop gathers around itself and stays in an almost spheric form – you're fine. If it just splats on the display, then you've got a case of worn-out grease-repeller. A more extreme test would be to pick up a marker and draw a line across the glass. If the oleophobic coating is still there, the line will erase itself, as the marker's trail breaks up and forms many little dots, which can be wiped off the screen; if the coating is gone, the line will stay on the glass (as you can probably imagine, we don't recommend this test, but you can do it for research purposes if you happen to have an old device lying around).
If you are feeling paranoid about losing your screen's pristine feel, or if it has managed to shed its oleophobic coat for one reason or another, you can always use a screen protector. Yes, many users have a poor experience with the films, as they tend to smudge up quite fast and feel unpleasant to the touch, but this is mostly because they have tried the cheaper ones on the market. The more expensive display defenders will have an oleophobic coating of their own and will be less annoying to the purists."
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Note:
To find the original piece, Google this:
"Oleophobic coating – what it is, how to clean your phone, what to do if the coating wears off"