no way to tell
(there's) no way to tell
It is impossible to be sure (about something or how something will unfold). It's possible that this maneuver could be hugely popular, but it could just as likely sink the company into financial ruin—there's just no way to tell. A: "Is this mushroom poisonous?" B: "No way to tell, unless of course you want to risk eating it."
(There's) no way to tell.
No one can find out the answer. Tom: How long are we likely to have to wait before the plane takes off? Clerk: Sorry, sir. There's no way to tell.