The phrase "wert onto someone" is an informal way of saying that someone is onto something or someone else. There are several synonyms that can be used interchangeably to convey the same meaning. Some of these synonyms include "onto", "aware of", "on the scent of", "on the trail of ", "in pursuit of", "hot on the heels of", "following closely", "cognizant of", "up to speed on", and "awareness of". Depending on the context of the sentence, any of these synonyms can be used to replace "wert onto someone" and convey the same sense of awareness or pursuit.