When it comes to discerning between two things, the phrase "tell the difference" is a common way to express this action. However, there are several synonym options available to add variety and depth to your language. For instance, you might say "distinguish between" to emphasize the importance of identifying key characteristics. Or, you could use "discern" to highlight the necessity for keen observation and careful consideration. "Differentiate" is another strong synonym that draws attention to the unique qualities that set things apart. Regardless of which synonym you choose, the key is to use language that accurately captures the action of telling the difference between two things.