I don't date / haven't dated, so I have no skill or experience with that - but with new people I meet there are all sorts of warning signals / red flags I get, mostly there are just different tropes or types of people:

• people who suffer from over-responsibility / Atlas complex - usually someone who was parentified by negligent parents growing up and now they have a dysfunctional relationship with a dependent partner / spouse or friends
• people who believe in ghosts, gods, religions, spirituality, etc. (whether New Age or organized religion) and are usually more likely to be victimized by scams, pyramid schemes, cult leaders, and pseudoscience health fraud; a lot of these people suffer from positivity bias and fail to think critically and recognize threats, particularly from within their trusted in-group
• drug abusers, for whatever reasons - alcohol and cigarettes are common because they're socially promoted and culturally tolerated; related: gambling, "sex addiction", risk-taking behavior (adrenaline junkies)
• workaholics - basically their whole life is work, most of their friends are only made at work, they make unreasonable sacrifices for work and have a hard time drawing boundaries, they might work on weekends, in the evenings, and basically are thinking about work all the time; often this is a form of escapism, and usually they neglect their partner / family / home life

I could keep thinking of more, there are a lot. Basically everyone I know has some major problem or another. I'd rather just stay home and interact with people anonymously over the internet, it's a lot safer.