My daughter (now eight) loves to ask me what words mean — including the ones commonly used in our household. She’s been doing it since she was three.
Whenever she does, she forces me to think about the words in a new light: explaining what a word means requires knowing other words to describe it sufficiently. So after a moment of racking my brain, I know whether I truly understand what a word means or not. If not, we go right to #1 on your list: Look it up!
Perhaps “Have someone ask you what words mean” is another way to expand your vocabulary.