How do I know when all "cleaner" is flushed from the cooling sys. during flush?

I used the word "cleaner" in place of "flush" to minimize confusion since the process is called flush and generally so is the chemical that's used to clean the cooling system. The Explorer is just coming up on 100k miles, and while I wouldn't describe the condition of the coolant as "bad", I haven't heard anyone suggest it would hurt anything to add a $10 bottle of VC-1 coolant system flush to ensure the system is as clean as possible. Plus, I don't expect to flush the system again for quite a while so I might as well do a great job this time. I appreciate the recommendation. From everything I've seen this seems like a fairly simple process that I should be able to learn to do correctly and perform on my own. You might ask, "if it's so simple, why are you asking for advice here?". I'm not afraid to try and learn new things. Also not afraid to ask questions (this is my first post and maybe I'm just on the wrong site?). I think a person should be able to do _some_ work on their own car, house, etc. without calling a professional every time. Over the years I've learned to accomplish the (generally) simpler tasks like brakes, plugs/wires, serpentine belts, valve cover gaskets, etc. and to do them correctly. It's saved me a LOT of money and I enjoy learning and accomplishing those tasks (always do them with my sons so they learn as well). That said, I do leave the more complex tasks (non-maintenance tasks that won't likely need to be repeated) to the professionals. Again, thanks for the recommendation. But, I'm confident I can learn to do this job on my own with a little guidance on one of the details. Thanks!