You should always immediately erase a drive before use. Any drive, any source.
Only problem with that is the microcontrollers and onboard program storage is large enough to put almost anything in these days. Flash drives are more powerful than my first computer.
Most reliable way is to write random data to the entire disk, then verify all of it. The disk can't fake storing it, as it could if you zeroed it.
I usually just dd something on to it, and see where it stops. I've never had one of the recirculating firmware drives that just keeps taking data but doesn't store it - but then again I don't buy 1TB flash drives from wish.com for 9.95$
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