Typing a checkmark with an alt code?

Alt codes only work with the number pad, not the number row along the top, so on a laptop without a numpad they won't do anything. That's normally why people can't get them to work. In Word there's also Insert > Symbol, set the font to Wingdings and the tick is near the end of the list (character code 252).

Alt+251 gives you √, which is the square root sign. Close enough for some people, but it isn't a real checkmark. In Word, type 2713 and then press Alt+X and it turns into ✓. 2714 gives the heavier one. Anywhere else, Windows key + period opens the emoji/symbol panel and you can search for check.