Same way you test code you wrote by hand. In-place and haphazardly, until you have it write unit tests so you can have it done more methodically. If it hallucinates a library or function that doesn't exist, it'll fail earlier in the process ; compilation).

I've used Claude to write code, and it is much harder to test that code than it is to test code "haphazardly" as I write it myself. Reason being, I can test mine after each new line I write and make sure that line is doing what I intend it to do. After Claude writes a whole set of functions, it could take hours to test all the potential failure modes.

BTW, if it doesn't take you hours to test the failure modes, you're not thinking of enough failure modes.

The time savings in writing it myself has a lot to do with this. Plus I get to understand exactly why each line was written, with comments I wrote, not having to read its comments and determine why it did something and whether changing that will have other ramifications.

If you're doing anything larger than a sample React site, it's worth taking the time to do it yourself.