You can not do that in the .skel file, as it's a binary stream, which can only be read sequentially, plus it has a dependency on the atlas as you found out.
The same is kinda true for .json files, as you first have to load the entire file content, then parse it into a JSON in-memory structure, and traverse that to get the slots. However, you can do that without spSkeletonJson, as it's just a run off the mill .json file. So, by reading the .json file, and then traversing the slots JSON nodes, you could achieve what you want.
That said, attachment information is not just contained in the slots, it's also contained in skins, so you'd have to traverse those as well.
What specific problem are you running into? From what I gather, you are trying to read a .skel file and that fails due to missing regions in the atlas? And you want to check that before actually loading a .skel? How do you create the .atlas files?
