Unlabled tileset conventions

I'm gathering tilesets for a new level, and keep encountering tilesets with no specification over whether they can be used or not.  What is the convention here for such tilesets?  Some have no owner, others have an owner and name, but don't specify anything as to whether they can be used or not.  Thanks.

The default for custom content that isn't declared as publicly available is to ask the owner; the policy on modifying content when the owner can't be contacted to ask for permission can be found here; I suppose it applies to mere use as well. If the owner isn't specified and cannot be determined in any way, I guess it falls into public domain.

Cool.  Thanks.  Now, I just have to figure out how in the world to get a hold of some of these guys...