The best way I have of knowing code is correct on all levels is convincing myself I would write it the same way.

Thr only way to be 100% sure is writing it myself. If I know some one reasonable managed to write the code I can usually take some shortcuts and only look at the code style, common gotchas etc.

Of course it wouldn't be the first time I made some erroneous assumptions about how well considered the code was. But if none of the code is the product of any intelligent thought well, I might as well stop reading and start writing. Reading code is 10x harder than writing it after all.