I wonder whether: liệu … (có) … không?

You know có … không? for a yes-or-no question. Put liệu in front and the question turns doubtful, like "I wonder whether …" or "will it really …?": Liệu mình có kịp chuyến bay không? Will we really make our flight? You're unsure, and often a little worried.

Nothing else changes: liệu, then the person or thing, then có … không. After không biết (I don't know), it's for wondering aloud: Không biết liệu mai trời có mưa không. I wonder whether it'll rain tomorrow. Liệu can be left out here: Không biết mai trời có mưa không says almost the same, and is more common in relaxed speech.

Liệu is frequent in the news, where headlines often drop the không: Liệu giá nhà có tiếp tục tăng? Will house prices keep rising?

Since you're asking about something nobody knows yet, the answer is usually a guess: Chắc kịp. We should make it. Khó lắm. It's unlikely.

Liệu only asks; it could go either way. Biết đâu states a chance and keeps it open, often a hopeful one, while sợ là expects the outcome you don't want: Em sợ là mình không kịp đâu. I'm afraid we won't make it. Chẳng lẽ is stronger: you can hardly believe something and don't want it to be true.