How to find the character breaking the file encoding?

[Marcin Borkowski]

Hello all,

I have a large UTF-8 file (about 1.5MB) to which I add stuff regularly.
Recently, Emacs started saving it as a binary file.  I suspect I somehow
inserted a non-UTF-8 sequence of bytes there.  Is there any way Emacs
can help me finding it (other than me manually bisecting the file until
I find the offending place)?

TIA,

-- 
Marcin Borkowski
http://mbork.pl