I finally proved my mother hates Americans after years of denial.
So my mom used to work for a small american company that had offices in the EU and was not impressed. After that she has been more annoyed by american accents, tv and films but always denied it. To test it, I talked her into watching The Office US with me. Put Pilot episode on and she wasn't impressed (to be honest, neither was I). She dismissed it and then I waited about half a year. Then I suggested watching the UK Office and she liked that. My mother has a s*** memory so i knew she would have forgotten the US one.
P.S. If you haven't seen The Office UK or US, both pilots have the same script.