Ascertain means to find out with certainty or assurance.
Ascertain is a verb.
Synonyms: certify, establish, find out. learn, verify, determine, detect, conclude, discover.
With one bite of the cake, the person ascertained that someone had mislabeled the salt as sugar.
After looking at their old map and consulting their compass, Mr.Ravi ascertained that they were lost.
The detective was able to ascertain quickly that the suspect had an affinity for chewing gum.