Have you been taken for an American in Japan ?

[Maciamo]

What makes you thin k the Japanese are going to be able to distinguish between a European and an American?
It is hard for me to tell between a German and a French person when they speak English.

It's very easy for me, but I am European. Some French people have "French" written all over their face. Same for many Italians and Spaniards. Language and culture is strongly expressed on facial expressions and in one's behaviour and style. Like for many other things (like the ability to put a name on a face, or recognise emotions in facial expressions) some people are more gifted than others. I am particularly good at telling nationalities or guessing what is a person's mother tongues in a multilingual country like Belgium or Switzerland. One part is innate, but practise is also necessary. The more one observes people the proficient one gets.

Most Americans are really quite easy to spot in a crowd.

It's hard to tell Scandinavians apart, or to tell a Czech from a Slovak, because their language and culture are very similar. But it's easy to tell a Swede from a Slovak or from a Swiss.