They're also easy to spot because instead of the OP getting more and more defensive, as usually happens when they're posting as themselves, they post less and less.
Eg, normal AIBU?
OP: AIBU to not want my MIL to have my PFB for Xmas Day?
MN: YABU.
OP: But my MIL smokes!
MN: YABU
OP: I meant, she smokes crack!
MN: YA still BU
OP: She smokes crack and breeds cockerpoos and has a 400inch plasma!
MN: YA still BVU
OP: You are a nest of vipers!
Whereas, reverse AIBU
OP: AIBU to demand that my DIL brings my PFG round for Xmas Day even though I like a few drinks before lunch.
MN: (over several pages) YABU, let her make her own plans
OP: smug silence
However, what often happens in a reverse AIBU is that the OP can't keep up the role playing and ends up giving themselves away, eg,
OP: AIBU to demand that my DIL brings my PFG for Xmas lunch.... despite the fact that I have a rampant drinking problem and will probably serve Greggs sausage rolls all day and will ignore my lovely DIL because I am jealous of her happy family life and svelte size 10 figure because I am a fat heifer who gives my PFG non-organic ham every time my DIL's slender back is turned?