I guess the question really is "how do you know, without looking deeply at his code" ( either because you don't have time, or because you're not a software developer). Which is a problem everyone has in every craft.

How do you know a plumber is a good plumber, if you know nothing about pipes ? Well, you won't until he's done : does it leak or not, and how long until it leaks again.

Now, on the other hand, if are a software developer, then simply look at the code in detail, and you'll know immediately.