Place the cursor where you want the checkmark
Go to the Insert tab
Click Symbol
Select More Symbols
Choose Wingdings or Wingdings 2 from the font list
Find and select the checkmark symbol
Click Insert
Click Close
Alternatively, type 2713 and press Alt+X
Alternatively, type 2714 and press Alt+X
Alternatively, use the Developer tab and insert a check box control
Alternatively, copy and paste a checkmark symbol from another document or website