Most astronomers agree that life is likely common throughout the Universe. While Earth is the only world known to have life, we know that life arose early on our world, and the building blocks of life, including amino acids and sugars, form readily. We also know there are countless worlds in the cosmos that might be home for life. But just because life is likely, that doesn't mean proving it will be easy. Many of the biosignatures we can observe can also have abiotic origins. So how can we be sure? One way is to compare our observations of a habitable world with other worlds in the system.