What is the meaning of I could see them sizing me up? - Question

Definition of I could see them sizing me up To “size someone up” means to judge them by observing them, to see what kind of person they are, or how qualified they are. Someone may size you up for different reasons - to see if you are nice, or if you are a threat, or if you have what it takes to do a certain job.
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What is the meaning of I could see them sizing me up? - Question about English (US)