If you use the same alloy for most cases and weigh it before sending you can compare the percentages to your alloy chart and have a decent idea.
I always kept my scrap separated and put it in an old crucible to melt and burn out the crap.
Being knowledgeable is important, your diligence here makes sense. A few years ago we were performing a very interesting analysis for clients that bought our alloy and refined with us. It's not that we were being exclusive to them only but because we had exact records of what they were buying in alloy over what period of time vs. what we were receiving back in refining material, we are able to conduct a "Health check" for them. It was very telling as we not only could calculate what % of alloy purchased was ending up in the scrap, we could also detect and point out areas of increased expense and help labs be more profitable. For example, if the scrap to alloy ratio was higher than normal and alot of it was grindings, we could highlight the fact that someone was being paid to grind more metal than normal.....Why? Maybe the waxer was waxing too thick, maybe the ceramist was asking the metal finisher to adjust the bridge because it wasn't designed properly, etc....Very interesting stuff....