How can you avoid harmful interference on an apparently clear frequency before calling CQ on CW or phone?

Before claiming a frequency, you must actively verify it is not already in use. Propagation changes or temporary pauses in a conversation might make an ongoing contact temporarily hard to hear, so just listening is not enough. The universally accepted procedure is to ask if the frequency is in use and then identify yourself. On phone, simply ask 'Is the frequency in use?' followed by your call sign. On CW, transmit the Q-signal 'QRL?' (which means 'Are you busy?' or 'Is the frequency in use?') followed by your call sign. Options suggesting you just listen for two minutes or send 'test/V' are incorrect because they fail to actively ask the occupants if they are there. Sending 'QSY' is also wrong because it is a command telling someone else to change frequency.