AFAIK, one can only find out if --xlevel was used when it actually became active, i.e. when the encoder actually used it to prevent one of these internal clipping errors. There's no way to find out whether or not the user specified --xlevel if the option was not used by the encoder. (I hope you see what I mean, it sounds a little dodgy)

If you take a WAV file that doesn't provoke internal clipping errors (like 99% of all files you'll come across) and encode with and without xlevel, you will see that the MD5 sums of the created MPC files match. I assume from this that the encoder doesn't store any information about the --xlevel switch being used unless it was actually needed.