欲しい and たい is used only for the speaker's own feelings/emotions, whereas がる is for the second/third person's.
When たい expresses the second/third person's feelings/emotions, the guess, hearsay, question form or like that is needed.
日本に行きたい。
I want to go to Japan.
彼はいつも日本に行きたがっている。
He always wants to go to Japan.
彼は日本に行きたいらしい。
He seems to want to go to Jaoan.
彼は日本に行きたいそうだ。
I heard that he wants to go to Japan.
日本に行きたい?
Do you want to go to Japan?