python - How to find a non-ascii byte in my code?

While making my App Engine app I suddenly ran into an error which shows every couple of requests:

    run_wsgi_app(application)
  File "/home/ubuntu/Programs/google/google_appengine/google/appengine/ext/webapp/util.py", line 98, in run_wsgi_app

    run_bare_wsgi_app(add_wsgi_middleware(application))
  File "/home/ubuntu/Programs/google/google_appengine/google/appengine/ext/webapp/util.py", line 118, in run_bare_wsgi_app
    for data in result:
  File "/home/ubuntu/Programs/google/google_appengine/google/appengine/ext/appstats/recording.py", line 897, in appstats_wsgi_wrapper
    result = app(environ, appstats_start_response)
  File "/home/ubuntu/Programs/google/google_appengine/google/appengine/ext/webapp/_webapp25.py", line 717, in __call__
    handler.handle_exception(e, self.__debug)
  File "/home/ubuntu/Programs/google/google_appengine/google/appengine/ext/webapp/_webapp25.py", line 463, in handle_exception
    self.error(500)
  File "/home/ubuntu/Programs/google/google_appengine/google/appengine/ext/webapp/_webapp25.py", line 436, in error

    self.response.clear()
  File "/home/ubuntu/Programs/google/google_appengine/google/appengine/ext/webapp/_webapp25.py", line 288, in clear
    self.out.seek(0)
  File "/usr/lib/python2.7/StringIO.py", line 106, in seek
    self.buf += ''.join(self.buflist)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xd7 in position 208: ordinal not in range(128)

I really have no idea where this could be, it only happens when I use a specific function but it's impossible to track all string I have.
It's possible this byte is a character like ' " [ ] etc, but only in another language

How can I find this byte and possibly other ones?

I am running GAE with python 2.7 in ubuntu 11.04

Thanks.

*updated*

This is the code I ended up using:

from codecs import BOM_UTF8
from os import listdir, path
p = "path"

def loopPath(p, times=0):
    for fname in listdir(p):
        filePath = path.join(p, fname)
        if path.isdir(filePath):
            return loopPath(filePath, times+1)


        if fname.split('.', 1)[1] != 'py': continue

        f = open(filePath, 'r')
        ln = 0
        for line in f:
            #print line[:3] == BOM_UTF8
            if not ln and line[:3] == BOM_UTF8:
                line = line[4:]
            col = 0
            for c in list(line):

                if ord(c) > 128:
                    raise Exception('Found "'+line[c]+'" line %d column %d in %s' % (ln+1, col, filePath))
                col += 1
            ln += 1
        f.close()

loopPath(p)

Answer

Just goes through every character in each line of code. Something like that:

# -*- coding: utf-8 -*-
import sys

data = open(sys.argv[1])
line = 0
for l in data:
    line += 1
    char = 0
    for s in list(unicode(l,'utf-8')):

        char += 1
        try:
            s.encode('ascii')
        except:
            print 'Non ASCII character at line:%s char:%s' % (line,char)