Everyone so far is saying that you don't or shouldn't test components before assembly. But that's not true - it depends on what you are making and your tolerances and necessary paper trail and such.
My pick and place came with an on-the-fly component testing system consisting of, essentially, V-shaped probes and an extremely fast RCL meter tied into the controller. It could pick components and press them on these probes (using vision to accurately locate them) and verify the component value/specifications prior to placement.
It certainly slows down the placement quite a bit but depending on your application, it can be useful or even necessary. If you are making medical devices, military/space and such, then these processes might be required. But for consumer electronics, as said above, you just buy from a reputable manufacturer/distributor, ensure the parts are sealed and you can trust that the parts will meet their specifications as long as you use them within their specifications. Make sure you trust your assembler too! It's easy to the wrong kind of paste, the wrong soldering profile, bad handling techniques or do numerous other things which can cause high failure rates either during the assembly phase or early failures in the field.