From observations of mine plus some personal experiences;
A major red flag is when a woman is highly active on social media to the point where it is a regular part of her life. If she is constantly taking selfies, particularly thirst trap type photos showing herself off in a provocative way it's often not a good thing to see. I've noticed that women like this are not only narcissistic but their entire existence is based around the validation they can get from others, particularly men. Women like this are naturally going to get a lot of attention and will be bombarded by men. These women will also lead men on, as the more orbiters she can get the better. Pretty self-explanatory, I'd steer clear of women like this without hesitation, just a modern example of attention whoring.
Something else I've seen, though not quite as obvious as the example above, is if a woman overshares personal issues at a time where it is inappropriate. If you have been dating for quite a while it is normal to open up about some traumas from childhood, etc. However, if on the first date she starts talking about her psychological issues or brings up something like sexual abuse or trauma this is a very bad sign. Quite a while ago I had a first date with a woman and she randomly blurted out how she was sexually abused as a child, there was literally nothing in the conversation that would have led to that. Very bad sign.
Some more obvious examples;
- Has a lot of tatoos
- Has unusual/excessive piercings
- Unnatural colored hair (pink, blue, etc.)
I've found that with these above very often these women are not something you want around. I've noticed that the women with unnatural hair colors often seem to have legitimate mental health issues.
Other things I'd steer clear of would be party girls or women who have an alcohol problem or are drug users, this really doesn't need any further explanation.