Please follow the steps below to select all the cells that are blank:
Step 1: Click anywhere in the worksheet;
Step 2: Click the "Home" tab from the ribbon;
Step 3: Click "Sort&Filter" and select "Go To Special" from the list;
Step 4: Check "Blanks" in the "Go To Special" window;
Step 4: Click "OK" at the bottom.
All blank cells are now in different color. Please check how to fill cells with a background color if necessary.