For certain
Definition 1 of 2
Pro Tip 1/3Certainty Level
Used to show you are completely sure about your knowledge or belief. 
I can't say for certain if it'll rain tomorrow.
Pro Tip 2/3Stress Importance
Emphasize the absence of doubt in what is being stated or predicted. 
The test results will come out negative for certain.
Pro Tip 3/3No Need for Proof
Express strong conviction even when proof is unavailable. 
She said for certain that she saw a dolphin in the river.