I've got some FLAC files and I'd just like to know an easy way to find out what compression level they are (ie; level 5)

There's not an easy way. You can infer the level by block size and other file properties, but it's not straightforward.

Could the compression ratio give any clues?

If you know the FLAC version (which can be inferred from the header), you could decompress and re-encode into different compression levels until you get the nearest/exact compression ratio. Only a suggestion... far from the best method!! There is no general way of telling from the compression ratio what level was used as it is purely dependant on the source file.

I'm not even sure that would work if you have a different version of flac.exe, since it's different from version to version. If you knew the version it was encoded with and the block size, that might be as far as you can get, and that would probably just eliminate levels (e.g. 1 through 3) but not tell you which it is.

1. Check the header;
2. Find that version of FLAC;
3. Uncompress to WAV;
4. Re-compress WAV at all available compression levels to separate FLAC files;
5. Determine which filesize is closest to the original FLAC file.........

Easy - yes; Fast - not even slightly......