Magpul MBUS Rear Black Flip-up Sight - $54.99 shipped (Free S/H over $25)

'Tis an easy search on the web/youtube for how to spot fakes, but the simplest highlights are:

1- Should say "MBUS" molded into the side
2- Magpul logo on the top/front should be recessed, not raised

Newer fakes have figured out the first two, so also look for:

3- Flathead bolt to mount to rail, not hex-head
4- Should be clean of "flashing" (mold marks) and free of wiggle/off-kilter parts

Another way to tell is to compare to any existing sights you already have... or a friend has. Not every living person happens to have them, but I have four Gen2 rear sights as a basis of comparison, so I'm pretty confident that I'll be able to spot a fake. Magpul makes quality gear so if you get one and you just have that voice in the back of your head saying "this feels/looks like a piece of s***", you might want to look at it more closely and/or compare with a known real one.