Olivier, please forgive me if I am speaking out of turn in respect of your Fuji X-T1, but my Olympus OMD E-M1 cameras and most of my Nikon bodies have electronic "spirit level" functions buried in their menus.
It might pay you to trawl through the X-T1's menus and see if such options are provided by Fuji and select them - in my case when they are enabled they are displayed in the eye level view finder or the screen / or via Live View.
Having the functions enabled in my cameras has helped me to train myself out of this bad habit or tendency - in my case the camera goes down very slightly to the right side at the time of exposure and the horizontals are thus tilted slightly to the left in the resultant image.
End of confession father.