Unknown paper

In clean out a darkroom at schools that no longer teach analog photography, I found lots of paper of different sizes and most of it in only the black bag. I have used some of it. Some prints dried flat while others curled. In my research, I believe RC paper is best for my students. How can it tell what type of paper is in the bag when there is no bag or label?

If it curls after drying, it's fiber-based. If it dries flat, it's usually RC.

Your students will 'eat' either form. They might prefer the RC because it dries flat. Processing FB for archival standards takes a much longer wash than RC, but for practice prints made my students, it won't matter anyway. If you end up using (some) FB paper, just give the 'keeper' prints an additional wash at the end of class.

Other than the distinction between FB and RC paper, you won't be able to figure out much about the papers if you have no label, documentation etc. Doesn't matter; if it works, it works and it's good paper. If it's horribly fogged, just toss it or put it up for free here on Photrio so someone might use it for alt. process printing etc.

Fiber based and resin coated papers have a very different feel and are often easy to tell apart by looking at the back side. Fiber based should look pretty matte, while resin coated has more or less a sheen to it due to the coating. They also do tear in a different way and handle differently. Fiber based paper behaves more or less as true paper, resin coated is more plasticy.

I would take a small piece of paper from each bag and develop it. Firstly to test if it still shows decent whites and doesn't develop completely grey, secondly to get an idea what kind of paper and surface it has. When it is still usuable i would do a second test to find out if it is paper with variable contrast or graded paper.

To find out with make it actually is, would be a bigger problem. When you have access to a densitometer you could plot curves, but due to age and storage conditions they probably differ from the known papers.

I don't recall if any other manufacturer other than Kodak watermarked the back of their paper.
I concur with Koraks, RC generally dries flat and (especially older) FB papers tend to curl.