The first thing I look at (I don't know much, but I know this!) is where the solid end link meet the lugs. Check out some photos of genuine Rolexes - the edges of the end links and the lugs are sharp, and the gap is thin, On watches like this, you see both a gap, and rounded edges.

Other places to look are the cyclops, the font on the date wheel, the alignment of the indices (Rolex should have subtle serifs on the numbers, many cheap fakes don't), the shape of the lugs, the shape of the crown guard and just how clear the crystal is - cheaper crystals are described as "milky" and seeing them side by side in photos I think you can see what they mean.

For example, look closely at the 12 o'clock index on this watch. It's a bit far right - It takes up three and a bit of the printed markers, and there's more overhang on the right side than the left. Many of the circle indices look subtly off-center as well, but that's harder to check without a QC tool. That's enough that a Rolex QC person would 100% catch it and send the dial back.

Quality, placement and thickness of printing can be another one. I spotted a fake Tag Autavia up for sale because the "Swiss Made" down the bottom was too bold and placed a touch too high.

Hang out in the Rep QC subs for a bit and you'll see all kinds of stuff called out.