I moved to Mexico in May of 2017 and arrived at a city in the state of Guanajuato. I didn't have a car, so of course, I had to take taxis. One day, I got in a taxi with a friend who spoke Spanish as a native and did all the talking while I stayed silent. After we got to our destination, I noticed that the taxi charged her much less than it would have charged me for that same distance. That's when I realized that taxis had been charging me much more simply because of my accent in Spanish. After this, I tried to say as little as possible in taxis hoping that they would not notice I was from the USA.
Well, a year after living in Mexico, I can now speak in Spanish freely without worrying about getting charged more for a taxi. That's how I realized my Spanish and accent had improved!