Lay the rifle in a stable position in sandbags or on a rest, then without touching the rifle, look thru the scope at a target or distant object at the range you're shooting at, as you change your eye position away from center slightly, the crosshair should stay in the same position on the target, if it moves around, you're seein the parallax error, centerfire rifle scopes are parallax free at 100-150yds depending on the mfr, rimfire scopes are parallax free at 50yds, so a CF rifle scope isn't the best for a rimfire at 50yds unless it has an adjustable objective that can be set parallax free at any range. The only way to use the CF scope is to make sure your shootin eye is in the in the same identical spot(consistent cheek weld) each time you mount the rifle, otherwise the parallax error will show up in your group size because the point of aim isn't identical each shot.
Tim