I generally sight from the butt end of the guitar towards the neck. There should appear to be a slight (maybe 1 degree) back angle from where the neck joins the body to the tuning head. This is NOT a backbow to the neck, but a clearly seen back angle to the whole neck from the 14th fret (the planes of the body top and neck).
You can also carefully lay a straight egde down the top to see if the neck appears to fall away a bit as it leaves the body joint and goes toward the head.
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