The way I see it there are a few different levels:
1: You have heard something, you aren't sure what it was, but think it was probably game, you know where it is coming from, but can't see anything with your eyes.
Rifle scope is not appropriate in this case. Binocs, spotting scopes, eyeballs, or let it walk.
2: You are unsure of what you are looking at, if it is game in the first place. You would have no idea regarding the legality of the game if it is indeed game.
Rifle scope is not appropriate in this case. Binocs, spotting scopes, eyeballs, or let it walk.
3: You know without a doubt that it is a game animal (the type you are hunting for) and believe that it is an animal that you wish to take, but it requires close observation to be absolutely certain (you have a buck tag an need to verify that the antler on the deer is over a certain length).
Ideally this is done with binocs or a spotting scope. It can be done with a rifle scope if no opportunity to use binocs presents itself. The firearm should have the safety on, and fingers should not be in the trigger guard.
4: It is definitely a game animal, and it is definitely legal game for you to take but you wish to look at the game for detail, I.E. number of points, symmetry of rack, whatever it is. Or , for example, if you have a deer tag that allows you to take both antlered and antlerless deer, to determine if the animal that is undoubtedly a deer, is a buck or a doe. Also think of situations where the party you are hunting with has a tag to shoot this game, but you personally may not hold this tag and may not want to use the party's tag on certain game.
A rifle scope is fine in these circumstances. The firearm should have the safety on, and fingers should not be in the trigger guard until you decide if/that you wish to take the game.
5: You are observing dangerous game, and the animal is within 100 yds, or is aware of your presence.
A rifle scope should be used to observe game in these circumstances. The safety should be on, and fingers out of the trigger guard, but in a position that both of these can easily be changed.
6: You are certain of the game, certain of its legality, and you wish to take the game, and are waiting for "the shot."
A rifle scope is appropriate to observe the game. Safety may be off, finger may be in the trigger guard as appropriate.
7: You are certain of the game, certain of its legality, and you wish to take the game, and have "the shot."
Why else would you have a scope? Safety off, finger in the trigger guard, shoot!
All of these presuppose that you have a scope on your rifle, elsewise its all moot.