I guess trial and error is the answer?
In all 16 possibilities, it's still usually easy to spot uncompressed graphics. They look different from other types of data. I knew what it was supposed to look like, I know what jumbled tiles look like. So I scrolled through looking for a block of jumbled tiles, and then looked for those specific tiles.
Edit: Another option is to look for a tool someone wrote that already deals with Zelda tiles. (A quick search didn't reveal one, but I'd be
really surprised if there wasn't one somewhere.)
The last option is to use the debugger to find how the data is getting to CHR RAM, which is actually the only way to do it if the graphics are compressed and there's not already a tool. But you need to know 6502 assembly language for that.