Check Working Keys Using Ubuntu's Handy Utility

I get the odd laptop for repair with dirty, dust coated, smokey environment causing functional issues, like sticky or non responsive key presses.

Smoke is not good for electronics! It sticks to electrical boards as it is ionised, and carbon is a conductor so can short IC pins out etc.

Ubuntu has a really handy utility in the Applications window that proves to customers what keys work and which don't, under Text Entry:

If you click the tiny keys icon (visible next to the Caps Lock in the pic above) you get the onscreen keyboard that lights up when a key is pressed (or not!).

Ensure you have the correct keymap installed for your keyboard type.

I could really do with this in Mint also, but don't know what this app is called and if available as a repo install…email me if you know or what utility pack it may be in please! It may be:

sudo apt install indicator-keyboard

but seems for use only in Unity…shame.

Mostly, failing keys means a replacement keyboard, but as you have nothing to lose, keyboard removal then a wash in warm soapy water MAY remove accumulated skin residue, dirt and dried residue from fizzy based drinks opened near the laptop, etc. that has run down behind the keys, that can cause a key to be stuck on (beeping at boot etc.) but it is not guaranteed by any means, and I have never had total success in 3 attempts, at best just I got some keys working that weren't before, never a full repair.

Leaving the washed and shaken keyboard on a warm radiator for a few hours should dry it sufficiently to see if it works.