The phrase "who knows" is commonly used when someone is unsure or uncertain about something. However, there are several synonyms that can be used to convey the same meaning in different ways. "Maybe," "perhaps," "possibly," and "conceivably" are all good alternatives. "I'm not sure," "it's hard to say," and "I have no idea" are also options when someone is genuinely clueless. "Who can tell," and "your guess is as good as mine" put the responsibility onto the listener to speculate or offer their own opinion. Whatever phrase you choose, remember that clarity is important in communication, so be sure to use common language that is easy for others to understand.