How can you avoid harmful interference on an apparently clear frequency before calling CQ on CW or phone?
Why A frequency can sound clear to you while a QSO is in progress that you cannot hear, for example when you only hear one side of a contact or the other station is behind a skip zone. The courteous and standard practice is to ask whether the frequency is in use: on CW the Q signal QRL? means "is this frequency busy?", and on phone you simply ask, in both cases following with your call sign so your transmission is identified per 97.119. Then listen for a reply before you start calling CQ.
Watch out Listening alone, even for a couple of minutes, does not reveal a station you cannot hear, and the choice using QSY is backwards since QSY means "change frequency," which is what you would ask another station to do, not an inquiry.
QRL? = "is the line busy?" Ask first, add your call sign, then listen.