When you receive a message, BEFORE you QSL you should check the CK matches the reality. That's why it's called Check (CK)!
If you need time to do this, send <AS>. You will note that is why often when receiving messages, the professionals often say <AS> at the end, before they QSL. As they may need time to double check the count. However, with practice, you can end up counting (especially if you write 5 words on each line) and have that done at the end almost instantly without needing to ask <AS> but sending <AS> at the end is normal and no shame in it. Be sure to check the CK count matches BEFORE sending QSL.
If it does not match, send QTB. The meaning of QTB is "I do not agree with your counting of words; I will repeat the first letter or digit of each word or group." When posed as a question, QTB? means: "Do you agree with my counting of words?"
QTB is thus very useful. The first letter (or number) of each word group is then sent, and then it becomes easy to see where you (or the originator of the message) went wrong. Fills can then be used (WA etc) rather than having to resend the entire message.
