The best way to know for sure is to purchase a scope base called a “scope checker”. It mounts to your rifle and has two pic rails. You mount two scopes simultaneously, zero one, then dial the second one to the exact same point as the first one. So when you look through either one they are both on the bullseye. Now start shooting. If both are good, then they will both continue to be aimed at exactly the same location. If the both pairs of crosshairs begin to shift relative to each other, then the good scope is the one that remains at the center of the group. If both are bad, then they will shift relative to each other and relative to the group.
It can be difficult to pick up on small shifts particularly with lower power scopes, two imperfect scopes, and rifle that isn’t extremely accurate. Guys checking multiple scopes for competitive use often purchase a “frozen scope” to use as the known good scope, and they’re using a heavy rifle with a flat fore end in a heavy front rest to shoot the groups. That said, with a frozen scope, any shift of the scope you’re checking indicates failure regardless of the group, so you could definitely do that regardless of the rifle and rest.
