"Est ce que ma phrase est juste?"

" Est-ce que ma phrase est correcte ?" is fine if you mean grammatically correct.

You would write "Est-ce que ma phrase est juste ?" You could also say simply: "Est-ce que ma phrase est bonne ?" Often bon / bonne is used to mean "correct" in addition to meaning "good." "J'ai pris la bonne voie." = I took the correct route. Note that "juste" is also sometimes used to mean something like "exactly appropriate to the situation" or "especially pertinent." "Le mot juste" is the precise word needed to express a certain idea, for example. "Une phrase juste" could be a pertinent remark made by someone in response to an event. So if someone describes a sentence as "juste," they might mean not only that it is without errors, but that it responds to a situation in an apt or intelligent way.