1. Because thieves simply take physical hold of something, instead of paying, or asking, they're touchier than non-thieves. So, if someone tends to touch people, and/or things without getting permission first, it let's you know that they are thieves.
2. A held-back thief will reach for what/who they want instead of actually touching it/them. They reach to call dibs, quickly, before someone they can't win a physical fight over takes it first. Or, they reach because they want to simply take it for themselves now, but they realize they'll be caught, and won't get away with it, so they stop short of taking it.
3. A thief who has learned neither to touch it, nor to reach to touch it will still eye it.
4. If they have an accomplice, they will gesture at the desired item/person to the accomplice in some way, whether overt, or covert.
5. If they usually have 1+ accomplices, they might gesture at it as though for an accomplice without one around, out of habit.
6. A very skilled thief will have learned to question, or to research, or to handle without trace of it without you noticing before they steal.
7. The more skilled is a thief, the better at reading other thieves. But, non-thieves, too, can learn this. Pay attention to people's eyes, gestures, reachings, fixations in conversation, etc.
8. A thief might give themselves away by having more focus on what/who they wish to steal than an average person would in conversation.
9. "Experts" think mirroring someone, or something is a flirtation. They are wrong. If you're mirroring someone, you are at least subconsciously imagining that they're you, you wan to steal from them something intangible, such as a personality trait, or a skill. If you mirror the person only with a specific item, you are imagining yourself as using it instead of who is using it currently. This can be a sign of desire to steal something, whether tangible, or not.
10. Only thieves declare dibs upon something, or someone instead of respecting the proper ownership.
The touchier; reachier; lookier; gesturier; mirorrier; item, person, or animal fixated in conversationier; and/or the dibsier the person is, the more thieving, too, and the more they want to steal from the people with whom they're like this.
If the person asking your size, or the item's size clearly won't be buying/making you a gift for why, and isn't about your size (so might be curious if you are the same size), they want to steal it.
If they ask the value of the item, or the person to another person, they might want to steal it to sell it, or to steal him/her for ransom.

Here's an example.
A non-thief who wants to know if you're the same size as someone, or your size for someone to buy/make you something will ask you.
But, a thief might grab your shoe, and look for the size written on it.
A thief that's learned they can get caught as a thief if they touch it without permission might go to handle it, and stop themselves.
A thief that's next learned they can be caught by reaching for it, will just look. They will look, because they want it.

A more skilled thief will either simply handle the item/person, knowing they can smoothly get away with it, or they will look at it.
Clumsy thieves will touch without charisma, and upset average people.

If someone touches your stuff without permission a lot, they plan on taking your stuff for their own. It might be the basis of your relationship with the thief.
If someone touches you a lot without permission, they might plan on sexually abusing you. It might be the basis of the relationship, no mater how much they fake it as platonic.

Only parents, and those entrusted by parents should ever be touching you without permission, and they should only do so for parenting reasons, such as to carry you as a baby.

The only time someone should be touching your stuff without permission is when it's to save you, or it for your sake. For your sake, not the item's.

If the thief tends to gesture at you, or your things, they usually have an accomplice around you that's in on the plan. It's more dangerous than if there were just one thief.

Also, if the thief tends to gesture at people, or things, they're in a gang.

If you are surrounded by thieves, either it's just a bad location, or they're up to something concerning you.

A thief trying to steal from another thief, might bluff by gesturing at things, or them to others that are not accomplices to make them think they're outnumbered.

If the thief is showing skill, and confidence, but is being overt, giving themselves away, they are up for physical confrontation, and they might want it. Those thieves you probably shouldn't fight with, unless you think they're bluffing. If you think maybe they're not bluffing... Tell Gambit I said hi.

All non-thieves do respect the lawful ownership of something, or someone (such as your self ownership, which you do have if you're not a slave.)

That American "experts" think that it's normal to do those behaviors proves that America is a kleptocracy.
That Americans don't have a natural right status to their own hair, bodies, clothes, etc., proves that America has slavery. That the U.S. is not officiating the slavery, and just takes it from nowhere without a lawful type of basis, such as in the case of debt slavery, proves, too, that America is a kleptocracy.
Your boss, and the school staff have no right to dictate what you do, or don't wear, or what you do or don't do with your hair, or skin.