I immediately assume there's something terribly wrong with anyone who's fond of me.

People who pick fights in fuzzy socks. JK

People who talk nasty to me about other people behind their backs, it makes me wonder what they're saying about me when I'm not around.

I agree with finding people who are overly optimist or sugary as being worthy of suspicion. If their smile doesn't reflect in their eyes you know it isn't real. Also, it may sound bad, but people who are overtly religious in public or perform virtue signaling - if they need to convince you that they are good people they probably really aren't.

The way you use "leery" covers a lot of territory. One reason for being leery is fear that the other will not uphold your confidence. I once worked with a lady who, if you told her anything, would tell others. We joked that the three primary means of communication were telephone, telegraph, and "tell MdGraw." Another thing is insincere language -- flattery, excessive gushing, etc. Another is outright lying or deliberate distortion.

All of those would give me cause to examine what they say very carefully and to be careful about what I say.

Actually, there are a lot of things.
But may be different forms of hypocrisy are prominent.

Confederate flag-bearers.

If they can't look you in the eye

Salesmen Jesus, o my fucking god, salesmen I can spot them a mile away before they open their lying manipulative mouths.

There are good people who sale things, but the real salesman is a ....ok I'm done. My blood pressure is up.

Hygiene, racist tattoos, perpetual drunkenness, lack of perception, reduced intelligence, voluntary stupidity, no moral compass

people who presume things about me when they don't know me

@silvereyes I so agree with you about overt optimism and over "sugary" speech. lol

That "I'll pray for you" or similar sets off my alarms.

By how they treat others. Children, animals, and people who have nothing to offer them in particular.

I'm big on eye contact. If someone averts their glance from me I immediately become suspicious and wary.

1. Talking about people behind their backs.
2. Cocky/arrogant attitudes

I'll think of more soon I am sure...

Living in Arkansas, I'm pretty much always on guard, but then again, most people in all sorts of places are basically shitty.

I get a bad vibes from people who do not like animals .Excessive use of profanity . People with hypocritical tendencies,People exhibiting Paranoid behavior

I go by the vibes I get off of them. Like I have a sense for when someone's not a good person just by looking at them. The first time I saw my uncle's fiance I did NOT like her. I didn't get any good vibes by looking at her picture on Facebook. Met her and she seemed sweet and stuff. But then her true colors showed after they got married. She doesn't let him talk to any of the females in our family, not even me, and etc. I knew I shouldn't like her.

Too negative kind of people, I just get rid of. The rest are on time managerment in degrees.

Avoids eye contact… Verbal statements trail off in volume… Overly enthusiastic. Quick to agree..

I feel very uncomfortable around people who judge with very little information and gossip about those they work or socialize with.

Being human

I get leery of someone when they start bad-mouthing other people. Especially if, when faced with that person, they act as though they love them dearly.

Had an ex wife like that; should have known better. There is a lot to be said for the idea of 'if you can't say something nice, don't say anything at all'.

If they breathe, I'm leery.

Hum more from a survival instinct these are a few things that automatically make me stand off-ish.
All these are based on past bad experiences.

Religious talk, references people who have lots of pictures of Jesus.
People with lots of guns.
Guys with confederate flags on their trucks.
People who spred drama or rumours.
Conservatives.
Nationalist.
Guys that touch with out permission.
Anyone who perpetuates rape culture or defends it.
Anyone who believes, in flat earth, chem trails, all kinds of conspiracies.

There are a lot more but these come to mine.

Silvereyes, I agree 100%!!! I'm afraid of people who are ALWAYS so 'sweet' and 'loving' with others. It's exactly what you found out about this person that makes me leery! I have personal experience with a woman who presented one face to everyone else, but let me know how disagreeable she truly was!