I realized that people in America would not be able to distinguish between the Japanese tourists and the Japanese-Americans based on their physical appearance.
I realized this when someone asked me for directions while I was in the States.
ご質問ありがとうございます。
①
"I realized that"＝「私は～だと気が付いた」
"people in America would not be able to distinguish"＝「アメリカの人たちは区別ができない」
"between the Japanese tourists and the Japanese-Americans"=「日本人の観光客と日系アメリカ人を」
"based on their physical appearance."＝「見た目では。」
②
"I realized this when someone asked me for directions"＝「誰かに道を聞かれたときに気が付きました」
"while I was in the States."＝「アメリカに私が居たとき。」
ご参考に！
アメリカ合衆国 
