"Saikin isogashii desu ka" when do i know is it mean "Have you been busy lately" or " i have been busy lately"

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it means "Have you been busy lately?" because there is "ka" at the end of the sentence.
if there is no "ka" it will be "Saikin isogashii desu" and it means " i have been busy lately".

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if there's "ka" at the end of the sentence, it's a question.

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ka is the key to this. If it's there, then it's a question, if not, then it's a statement.

As for the subject of the sentence (which is usually invisible), you have to work it out from the context.

Evden çıkmadan dil öğrenme fırsatını kaçırmayın. Deneyimli dil eğitmenlerimizden oluşan seçkimize göz atın ve ilk dersinize şimdi kaydolun!