How to find out the defect which occur in white box testing?

First things first!

Make sure you have all the test requirements setup (test plan, Test cases and tools required)

Then go ahead and create a checklist with each stage of testing or a little complex and100% results are through Traceability matrix.

If any defects exists can see else the code is good enough to be moved to prod.

Hope this answers your questions.

Thank you,

AH