The Spell Checking... command can be invoked 2 ways:
Spell Checking your document is controlled through a dialog box.

In this example, the mispelled word KWord found, was youve. It has only one suggestion you've.
There is a progress bar below the list of suggestions which tells you how much of the document has been spell checked.
From here you have eight options:
Replaces the current word with the suggestion. Only replaces this occurance.
Replaces all occurances of the current word with the suggestion.
Do not make any changes to this occurance. Ask me if this word appears again.
Do not make any changes to this or any other occurance. Do not ask me about this word again.
When the entire document has been checked, KWord will return you to editing your document.
If your document does not have any spelling errors, KWord does not provide you with a dialog box. When you start the spell check, it will proceed to check all of your document against the dictionary, and then exit the spell check dialog box. With short documents, you may miss the dialog box. KWord has spell checked the document!