What is the proper Q code to use to see if a frequency is in use before transmitting on CW?

QRL? asks "Are you busy?" or "Is this frequency in use?". It is standard practice on Continuous Wave (CW) or Morse code to send "QRL?" before initiating a CQ or conversation to ensure you do not interfere with an ongoing QSO. If the frequency is occupied, a station will usually reply with "C", "Y", or "QRL" to indicate yes, it is in use. The distractors represent other Q codes: QRV? means "Are you ready?", QRU? means "Do you have anything for me?", and QRZ? asks "Who is calling me?". None of these are used to check if a frequency is clear before transmitting.