Being on to something is a well-known phrase in English that means to have discovered or become aware of something. There are many synonyms that can be used in place of "being on to." These include, but are not limited to: catching on, figuring out, getting the drift, grasping, comprehending, realizing, understanding, and perceiving. Each of these synonyms conveys the same idea that one has become aware of something that was previously unknown or hidden. It's important to have a strong vocabulary in order to communicate effectively, so including a variety of synonyms for common phrases like "being on to" can make one's writing more interesting and dynamic.