When I don't have a speaker cab connected to one of my tube amp heads, I use a piece of blue painters tape over the input, with "No Cab" written on it.
I also keep my gear unplugged when not using it so it doesn't get fried. My neighbor's house was hit by lightning several years ago, and it fried everything that was plugged in...another neighbor's house was struck by lightning and the roof caught on fire.
I'm more forgetful as I age, so I try to limit my exposure to it...now I may need to label the amp output and cabs, "Be sure cable is all the way in"