Select the cell where you want the check mark
Go to Insert
Click Symbol
Choose a font such as Wingdings or Segoe UI Symbol
Find and select the check mark symbol
Click Insert
Click Close
Or type 2713 and press Alt+X
Or hold Alt and type 0252 on the numeric keypad in Wingdings font
Or use the formula =CHAR(252) with Wingdings font applied
Or use Insert > Checkbox if you want an interactive check box