Hm, i'm weird, so this is how my red flag list goes:
a) all the right answers, right away. But i mean right to the dot, no 'buts', nothing. all mechanical-like.
b) after we exchange first names, putting my name at the end of each effing sentence. reminds me of bad times, so yeh, i know it's not a red flag for normal people, but it just wants me to back the hell away.
c) the obvious, outright insults most wannabe dominants use. but those are a red flag for most subs, i guess.