Generally speaking they are tested on running the engine. if they rattle, they are probably clogged with crud. You may be able to clean them with a lot of patience and a strand of copper wire and carb or brake cleaner.
Putting them in a vice is a good idea as it will force the crud out, but just be careful as you could also blow the seals. I know people used to do this on Ford CVH engines as a matter of course, but it made them a buggar to start as oil pressure had to sort them out before they would start and run properly. This because every time it was switched off, the oil would leak from the tappets. I don't know what the oil flow to the tappets is like on a Peugeot engine.