A little pocket flashlight is always useful, put one on your key ring. Doesn't need to be intense.
Shine your light along the surface, you should see luster or sheen from the Treasury printing on the surface, while cleaned creases will be lackluster, dull lines. There might be raised fibers in the creases, as well. You backlight the note and ironed-out creases should show up. Also, there will be some (not always) residual dirt in the creases. Look for the soiled lines across the note. Check the corners for rounding, as well as trimming. The margins should not be less than straight. Check the paper of the note for 'soft' lines, where the crease has been washed and ironed out, the paper in the crease will be slightly abraded or raised, the rest of the note will retain it's 'hard' surface, to a great extent, although washing will soften there, also, but less so than the creases. There should be some luster to the paper on an unwashed surface.
The paper of the note should be natural, and show a slight bend from gravity when held horizontally at one margin. A starched note will be straight and stiff, not at all naturally yielding.
I have bought some doctored notes with good eye appeal, but I got them for not much more than their original grade value, I won't pay an AU price for a doctored note, but will pay VF plus a bit more if I want the note. When the price of an AU or UNC note is several hundred dollars and up, I don't mind saving good money on a cleaned example, if it means having a note in my collection or no note at all.
Unless the vendor admits to the note being doctored, and comes down to my price, there's no deal. I don't want to reward this activity beyond a token amount. I consider cleaning and pressing to be acceptable solely for eye appeal and a quick sale at the original grade price plus a small percentage for the labor of cleaning. When the vendor raises the grade and price, he's dishonest and should be avoided unless they admit to the condition and drop the price.
ebay vendors who state 'the scan is the grade' are common vendors of high-grading prices and cleaned notes.
There are lots of very good reasons for
TPG graded notes, and cleaning and pressing is one of them. What should be considered as an archival practice has become, frankly, theft. When I started collecting, I bought a few cleaned items in uninformed negligence. Since then, I've learned from those mistakes and hopefully have the right opinion about the practice, both in knowledgeable buying strategy and ethic.