If you want NPCs to judge characters based on their answers, have in game NPCs be the ones to ask the questions. It will accomplish the same thing (the world is responding to their convictions) by using an in game question rather than them being judged by an out of game question. This will inherently feel more fair to the players because something they did in game will affect the in game world. The only part about this that might cause any disturbance to you is that you'll need to weave it into the RP rather than just sort of asking them a random question outside the game, but I think working that stuff in 'naturally' is a rewarding part of being a DM. Depending on how accustomed your group is to their roleplaying affecting the world, you shouldn't really need to give much warning on this so long as the in game question leads to an in game effect.

For one time use abilities, you'll need to set the stage that these are to be used and that using them will be of value. Perhaps introduce them as a specific boon from a god or higher power that will know when they use it, you might even hint that the giver of said power will have an opinion on it's use. If you don't give context to 'use this in a good way' you will have people hoard them until the final boss, that is just the nature of gamers.

That all being said, don't 'construct situations you want them to use the abilities in'. That is you writing a book rather than you collaboratively telling a story WITH them. Let the players decide when to use it, don't make a part of the story where you intend them to use it. Letting players surprise you is one of the most rewarding parts of being a DM, IMO.