There are a number of steps in your "Am I finished yet?" checklist.
As you worked through the lessons, you compared your sheet with the illustrations. So you know if you got all the data entered and if your formulas produced the right results. You can look at the column and row sizes and the number formats in the pictures, too. You had it easy!
When you are working on your own, you must carefully check as you work. It is hard to be sure after you have several screens of data!