What subtle red flags have you learned that might not be so obvious to others?
If all of their exes are bad/at fault for the relationship failing. This is a huge one because not only does it suggest they were the problem but it also demonstrates their lack of taking accountability or having self awareness.
If they don't have any friends. This is an odd one because people can just be introverts or have a genuine reason why this is, but it can also mean they don't have any friends for a reason. Most people have at least 1 friend in their life, if they have none question why this is.
They treat their family poorly. I have always noticed that a person who treats their family poorly is an indication you could be on that list once they get comfortable. This is usually the parents and unfortunately, parents don't just cut their child off so easily so they just put up with it.
Everything wrong in their life is someone/something else's fault. An extension of number one, lack of accountability or self awareness. I.e didn't get a promotion because of x, can't save any money because of x or that car accident happened because of x. Some times there is a valid excuse, but not for every little thing that goes wrong in their life.
Not having a job, hear me out. Some people have a valid reason, disability, illness, childcare etc. But if they have gone for years without getting a job with excuses that are vague, changing all the time and have no evidence or weight to it then it demonstrates their lack of desire to be an equal partner or work towards any kind of future. You may be working to support them, most people can work and there is a job for most people. It may be laziness.
I could go on and on but can we share together? I wish I knew these when I was in my 20s!