it is impossible to guess.

There's no telling how much damage has been done to young people by these “designer drugs.”

Even when we don't let them loose, there's no telling what kind of trouble the puppies will get into.

Related vocabulary: you never can tell

• get the jump on
• get the jump on someone
• get the jump on something
• get the measure of
• get the measure of someone
• get the measure of something
• get the message
• get the munchies
• get the nod
• get the picture
• get the push
• get the runaround
• get the sack
• get the shaft
• get the short end
• get the short end of the stick
• get the thumbs down
• get the thumbs up
• get the upper hand
• get the wind up
• get the wind up someone
• get the wrong end of the stick
• get through
• get through to
• get through to someone