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According to Ken Thompson's lecture at his Turing Award, you can't:

Practically speaking, you can verify that it's unlikely that there is no backdoor, but it'd take a fair bit of effort.

Impractically speaking, you could singlehandedly reimplement decades of computer engineering starting with a pencil, paper, and a bucket of sand.

From NAND to Tetris in 12 steps

I prefer placing rocks in an infinite desert to simulate the the flow of data through a computer myself.