A lead sound will usually have more of a short, sharp attack, and be used for playing melodies rather than long, drawn-out notes or creating soundscapes as a synthesizer creating a pad sound would. There aren’t super strict criteria for what makes up a pad sound, but there tend to be some subtle differences between the two. If you’re unsure, cycle through some of the synthesizer presets that have come with your synth and listen to the differences between a pad and a lead sound.