GPS QA during manufacturing
There is no free lunch here. You need a solid test plan. This is an area well worth investing in, because once you develop a reputation for shipping shoddy products, it's pretty much impossible to shake it.

To a large extent, you need to be able to trust your component suppliers, and focus on testing only for the types of errors that occur during the assembly process, including wrong/missing components, poor soldering, etc. Visual inspection is one method that can be applied here, but there are also automated electrical tests that can be used to find a large majority of the potential problems. But if you are using unreliable suppliers, then you must invest more resources in incoming inspection and test.

But there's no substitute for a full-up functional test at the end, at least on the initial batch. If you find that your failure rate is low enough, you might consider only spot-checking subsequent batches, or dropping the functional test altogether, but you need to balance that against the risk and cost of the resulting customer returns and dissatisfaction.