"Be onto someone" is a phrase used to indicate that someone has figured out or become suspicious of someone else's activities or intentions. There are many different ways to express this concept using synonyms. For example, you might say that someone has "caught on" to someone else's actions or that they have "seen through" their facade. Other possibilities include "figured out," "discovered," and "uncovered." Similarly, you could describe someone as being "alert to" or "mindful of" someone else's motives. No matter how you choose to express it, the idea behind being onto someone is always the same: someone has become aware of something that they were previously unaware of.