no flies on someone

chiefly British, informal

- something that you say which means that someone is intelligent and able to think quickly

Cambridge Dictionary

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ORIGIN

Did you ever notice how much flies like to hang around farm animals in particular? Cows can't do much of anything except swish the flies away temporarily with their tails. And because cows are not especially active, the flies can make themselves at home.

In contrast, animals that are active and clever enough have no flies on them. And it's precisely this concept that gives us the expression "no flies on someone" which describes a person who is not easily deceived or fooled, because they are smart and are always aware of their surroundings.

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BY THE WAY

And if you don't find flies on animals, maybe you will find a "fly in the ointment (die Salbe)" an expression from the Bible: Ecclesiastes 10:1: As dead flies cause even a bottle of perfume to stink, so a little foolishness spoils great wisdom and honor. The modern sense refers to something small and unpleasant that spoils something positive, such as "His presentation was structurally good, but the fly in the ointment was that he talked too fast."

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Practice OWAD in a conversation:

"There are NO FLIES ON Jane, she'll go far in her career."