[cj]

“I trust that you believe your code is correct, now let’s double check”.

Or maybe the proverb needs to be rewritten as “feign trust and verify”

[Lerc]

Or assume good faith but, since anyone can make mistakes, check the work anyway.

That's a bit wordy but I'm sure someone can come up with a pithy phrase to encapsulate the idea.

[stavros]

"Trust, but verify"?