What subtle patterns in other people spell nothing good?
There are no 100% reliable cues, because to some degree the situation is anti-inductive. If it becomes known that "bad people do X, good people do Y", bad people start doing Y. For example, it is known™ that liars avoid looking into your eyes... and maybe it was true once in the past, but today all scammers and salesmen make sure to look into your eyes all the time.
That said, most people update slowly, so often you don't need to outrun the predator, you just need to outrun his typical prey.
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Time pressure is a red flag. When someone forces me to make a decision right now, that usually means they want me to make a choice I wouldn't make if I had more time to think about it or ask other people. Even if their arguments seem perfectly logical at the moment, that probably only means that they gave me arguments for one side (and are trying to prevent me from finding the arguments for the other side), and sometimes the arguments are built on false assumptions (that they are preventing me from finding out).
Even a small lie (that in a social conversation would be polite to ignore) is probably just a tip of a much larger iceberg. The person lying about having one drink is probably an alcoholic. The person with one false information (that you know about) in their CV should definitely not be hired. -- Yes, most neurotypical people lie once in a while, you don't need to make a drama out of it. The important parameter is "lies per minute". If you only know someone for 10 minutes, and they already told you a lie that you could notice, that is a huge red flag.
Promises only mean something when coming from people you have known for some time as reliable. Or when written in a notarized document. Otherwise, verbal promises mean nothing. A sociopath will flatly deny that they ever said those words. A neurotypical person will instead insist that you have misunderstood their words, you misremembered them, or you forgot some important context that completely changes the meaning.
"You and me are the smart guys, everyone else is an idiot" is what manipulators say, because it makes you feel good (if you are inexperienced), and allows them to dismiss any evidence coming from a third party, so they effectively establish themselves as the only source of truth.
Some traits of this variety are heavily offset by charm or charisma. How do you tell the difference?
If the situation is too good. That is, rather than the usual "there are always both advantages and disadvantages, but the advantages of X significantly outweigh the disadvantages", they insist that there are no disadvantages whatsoever and they make you feel guilty for even considering that possibility.
(Generally, "making you guilty for even thinking about X" is a huge red flag.)
If they explain a conflict they once had by spending most of the time attacking the accuser, and pay almost no attention to actually defending themselves (other than saying "that didn't happen" and immediately returning to attack the accuser).
Generally, you can find out more about people by asking someone who knows them. People are cautious around strangers for a good reason. That should probably apply more to charismatic strangers. (There is a logical explanation why an average person is lonely, but why is this charismatic person alone rather than with their friends? Is there a chance that they simply don't have friends, for a reason?)
I used to think I lacked situational awareness. A search for cures returned no direct method to improve it. This is because we can perceive the same situation, but differently. When we're interested in something (like the model of a friend's new car), we find it more often. It's pulled out of our visual input instead of dying off during denoising – like in frequency illusions.
To a large extent, attuning ourselves to more important things is "situational awareness". There may be other factors, like our basal arousal or how drunk we are, but variation in those seems to zero out in comparative daily importance.
Since it's about attention, we can orient our own eyes to see things differently – creating selective frequency illusions. Learning a new word can make it appear more often in media, since the sound has sudden meaning, but the effect can also be used as a form of proactive prevention. The more complex your environment, the more likely you don’t notice a sign before too late.
What signs have you learned to look for?
To offer some slight reframings to the original question and more specific categories: