Mark your earbuds with a fine paint marker
My earbuds are rounded and nearly identical left to right. They're also silver grey so the markings are heard to see. So it always took me a sec to make sure the the right one goes in the right ear.
So I got a fine point paint marker in white and put a little dot of white on the right earbud. The paint is very opaque and shows up even on black plastic. It's also durable and doesn't rub off easily like silver Sharpies, etc.
You should blow on it to let it dry (paint markers take a minute to dry, and you'll probably have to shake it up first).
You can also use the marker to mark which side is up on your old USB-A cords, polarized extension cords, etc.