So I borrowed a trick from my sister and wrote "RAG" in permanent marker on each one.
We also have old bath towels that have been given a new life as "dog towels". Not only are they useful for drying a wet dog, they also come in handy for car washing and big spills. I wrote "DOG" on these.
Now there will be no question which are rags/ dog towels and which are the good ones we use everyday!