The biggest red flags I have found are these two;
1) Secretive or obsessed with 'privacy'. Someone who makes the consiquences for talking about 'their' business (even if it's also YOUR business) unbarably emotionally high.
2) Someone who doesn't want people to know they are taken. I don't mean someone who is closeted about poly. I mean someone who doesn't want other people to know they are anything other than single.
With all my heart I will love and not fail,
With all my soul I will fly and not fall.