The word "suss" is typically used to mean understanding or figuring something out. There are a number of synonyms for this word, such as deciphering, deducing, comprehending, grasping, and apprehending. Other potential synonyms might include perceiving, discerning, realizing, and seeing through. All of these words share the common thread of suggesting a mental process of gaining insight or knowledge into something. Depending on the context, some synonyms might be more appropriate than others. For example, "comprehending" might be a good choice for more academic or technical subject matter, while "perceiving" might work better in a more abstract or intuitive context.