Everyone usually has a general idea of their own faults. They sometimes are even able to fix these over time.
However, it can be very hard to notice some of your deeper flaws. Sometimes, that's not necessary as there are a million other ways you can improve yourself.
A trick that I've been using is analyzing others. I ask myself; what parts of the other person do I like and what do I dislike. This includes physique, personality, general vibes, sense of fashion, makeup, hobbies, quirks, and how they communicate.
[This is just the surface level and should help you get a general idea of what to pay attention to]
One very important aspect that some people overlook, is that you do not want to be "perfect." A seemingly perfect person will appear very intimidating and is NEVER trustworthy. Thus having a few quirks is a very positive thing.
People also interact very differently with different people. With a slight amount of practice, it becomes very obvious when an interaction between two people is shallow and ungenuine. It's basically as if both participants are wearing masks to give off a better image of themselves. Those interactions can be very interesting to watch.
The easiest to spot has to be new/potential couples and business environments.
[There's also no need to over analyze a situation. That will almost certainly cause more harm than good.]