That's a really good question. Unfortunately the library is a little lacking in this area but thanks for bringing it up so we can do something about it. Essentially we've modeled the classes to match the map format as it appears in the XML. I realize this isn't especially useful when you're trying to query things in a natural kind of way.
In the mean time, let me explain how it works so you might be able to come up with a workaround.
The Tiled map format has a few different elements relevant to what you're trying to do. It's essentially structured like this:
Each map has multiple tilesets and multiple layers. Each layer has many tiles.
Each Tile has a Global Identifier (GID) and each Tileset Tile has a Local Identifier (LID).
A Tileset also has a First Identifier (FID).
I believe (from memory) that you can calculate the GID by adding the FID to the LID. In this way, you should be able to figure out what Tileset Tile is used in each Tile of the map. Make sense?
That's about all I can tell you right now without looking into it a little deeper. I'll add it to my to-do list to make the API a bit nicer in this area. I'd be interested to know your thoughts on how you'd like it to work?