I have seen so many "eyeballed" home mounted scopes so crooked ...
Just "eyeballing" it might work. But there are a few other things that make a difference.
You use your eyeball but you should also insure the rifle is actually level when you do this.
One way is to hold the rifle or barreled action in a vice or bags or any method that will hold it without you touching it. Level it the action using a level.
Not touching the rifle look "through" the scope at nothing in particular ... you are not concerned with an image just the crosshairs, your eye can be behind the buttstock ... as you "eyeball" it try and imagine the bottom of the cross hair extending down through the center of the action... If the action is level and the crosshair does intersect the center of the action, your scope is pretty square. Step back and look at how crooked you stock may or may not be.
If you try and line a scope up "square" with a shouldered rifle, the fit of the stock and the "cant" of your head can cause it to be out considerably.
Another way is to sight on a distant object as a pole or edge of a building, while the action is level. If you don't start with a measured level action it is pretty difficult.