Most buyers buy an inspection and believe they have bought quality control. The inspection is the third and weakest of the three levers.
Before: definition
The specification and the golden sample define what « correct » means. Without them an inspector can only check for obvious defects, because nothing tells them what the product is supposed to be.
During: the only moment you can still change the outcome
An inspection while the line is running can stop a systematic defect after a hundred units instead of after ten thousand. It is the cheapest inspection there is and the one most often skipped, because it requires being organised before production rather than after.
After: confirmation
The pre-shipment check confirms what is in the boxes before they leave. Useful, necessary, and almost never where quality is actually won.
If you can only afford one, take the one during production. It is the only one that can still change something.