The simplest way to add a check mark is to copy and paste ✓, but documents have built-in options too.
In Word or Google Docs
In Word, type 2713 then press Alt + X. In Google Docs, use Insert → Special characters and search “check mark”.
Mac
Open the Character Viewer with Control + Command + Space and search “check”.
Which check to use
✓ is the plain check mark, ✔ is a heavier one, and ☑ is a ticked checkbox. Pick the one that fits your list.
Facts to verify: the Word Alt+X behaviour for U+2713 on the current version.