opažati

1. to notice; to observe
   Deca brzo opažaju promene kod roditelja. — Children quickly notice changes in their parents.
   Sve češće opažam da mi popušta vid. — I notice more and more often that my eyesight is getting worse.
2. (through the senses) to perceive
   Ljudi različito opažaju boje. — People perceive colours differently.

The perfective is opaziti; in everyday speech primećivati is more common.