english......../

有些少分別:

Have you been to Japan? 是指問者不肯定對方曾經去過日本與否。

Have you ever been to Japan? 是指問者在其心裡估計對方未曾去過日本的可能性高於去過。

以上是本人愚見。

Have you been to Japan? 與 Have you ever been to Japan?

於意思上大致相同。

ever在此句的用法只是用於加強疑問的語氣，是可有可無的。

Have you been to Japan→你有冇去過日本.......Have you ever been to Japan→你係唔係成日去日本

no difference

沒有乜野分別.....
只是多左一個字