Sam usually ignores Dean while they’re getting ready in the morning. Being guys, they don’t think anything of walking around with their shirts off (for a while Sam was self conscious of how thin he was, but he’s gotten much better with a steady diet, and figured Dean’s seen it all ever since he was injured in Taken). He’s well acquainted with many of Dean’s scars, especially the ones in his hands. Many of these might even be too small for Dean to realize he has, nicks and scrapes from repairing the Impala for all those years.