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！
I suppose it's hard for Americans to differentiate between Japanese tourists and Japanese-Americans just based on their looks.
この場合、次のような言い方ができますよ。
ーI suppose it's hard for Americans to differentiate between Japanese tourists and Japanese-Americans just based on their looks.
「アメリカ人にとって、見た目だけで日本人観光客と日系アメリカ人を識別するのは難しいんだろうな」
to differentiate between A and B で「AとBを識別する・区別する」
ご参考まで！
アメリカ合衆国 
カナダ 
