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There is no way to prove that random data is not actually encrypted data. This is a feature called plausible deniability

"Alright ma'am, you claim this here device contains encrypted data? If you don't mind, we'll keep you here until you provide us with the password."

and meanwhile in an alternate universe

"So you say that this device just happens to contain a bunch of random data? That sounds mighty suspicious. We'll keep you here until our technicians have taken a look at it. Get comfy, it'll be a while."

Does TSA ever inspect hard drives at the bit level?

Imagine being lectured on entropy by a TSA agent with an electron microscope.

If you care, do a final pass with zeroes.

(IMHO the very concept is obsolete in the era of SSDs & SMR.)