They make clear plastic connections that will hook to the black tank output so you can see what's flushing out. Flush until clear, but don't get too hung up about it, it's not that big of a deal.
There are people that never flush their black tank, after all you're just going to get it dirty again. The main thing to do is empty it before storing it. You can flush it then.
Use lots of liquid in the black tank so everything flows easily when emptying it. If, boondocking, you can do your dish washing in a dish pan and put the used dishwater down the toilet to both save room in the grey tank and put more liquid in the black tank to help everything drain well.
Bill