I wouldn't use a test piece, at least not routinely. What I'd do is link up your extraction to a failure alert system, e.g. an alarm or ideally an alarm and a belt stop if the vacuum fails to draw above a certain limit. I'd then do a routine check by switching off the vacuum or blocking the line in some way.
I suppose as a validation it's an option to put in a "test piece" of some kind but in reality what you want to suck out is likely to be small and vary considerably in density but as a validation you could find out what kind of contaminants your vacuum is likely to draw out.
If you do go down the route of having a test piece and providing you have metal detection later in the process, I would certainly make your test pieces metal detectable so you know you're not likely to then let them go with the product but I would say it's a procedure which is fraught with risk of contaminating the product (more risk than not testing it.)