How do you know if you don't have any?

As long as you are not dunking M4 into salt water it holds up fine. I have cut a lot of foods which are normally rough on carbon steels and it hasn't shown the first mark. I have been to the coast a bunch and on boats out on the salt water many times and I can't see anything but shiny metal on my M4. Go to the Spyderco sub-forum and check out some of the Gayle Bradley posts since it is uncoated M4. Many look perfect and even a vinegar bath doesn't have near the patina that O1 or 1XXX steels get.
With that DLC coating there shouldn't be any issue with rust unless it is put into chemicals or left in a wet place for extended periods of time. I think when more people get their hands on M4 and see how sharp and how easy it is to get sharp, these are going to be really popular. I'm actually surprised they aren't sold out yet.