For interview purposes, I build a LEGO Mindstorms framework to push the buttons and check that the correct LCD segments light up. And wait several weeks while

For interview purposes, I build a LEGO Mindstorms framework to push the buttons and check that the correct LCD segments light up. And wait several weeks while it runs through a sufficient number of iterations to give some assurance of correctness. For real-world purposes, I see if it turns on and gives me 4 when I do 2+2. If it fails, I buy a new one off Amazon.