We don’t want this to be a point of confusion, so the short answer is: Go to the “Sounds” page for your focal bird species in the Cornell Lab Bird Guide, e.g., American Robin, or your favorite bird app, and look up the various vocalizations that your focal bird species makes, which will be clearly listed as either a “song” or a “call.” You can hear the differences between the specific songs and calls your focal bird species makes.
Longer answer: It’s difficult to tell the difference unless you study the function of bird vocalizations and/or you have enough experience to differentiate the general context and quality of bird vocalizations. Fortunately, there are many bird experts who have this experience and knowledge who’ve made this information readily available through recordings of bird vocalizations available in bird apps and online bird guides.
A more thorough description of the differences between calls and songs can be found in numerous online articles, including on Wikipedia.
Listen over and over to the recordings of your focal bird species’ call(s) and songs on your bird app or the Cornell Bird Guide and see if you can match that to what you’re hearing in your backyard.
After you’re confident you have identified your focal bird species’ vocalizations accurately, demonstrate recognition to yourself by practicing picking out and/or recording those sounds consistently on different days and in different locations.