Target image can be cystal clear and in forcus, but still have a paralax error....in the high power scopes, the focus at close range is a good clue to paralax erro, but in lower power scopes it isn't.
To check, set the scope on a solid surface so it points onto a target (or a knothole...or a chalk X on a fence...whtever) without being touched...it sits there all on itsown. As you look though (not touching...no human influence), move yor head from side to side/up and down...if the cross hair intersection dances off the target point, that's paralax.