Look through the binocs at a distant object.

Keep your eyes focused on the distance, while moving the binocs away to arms-length. Or if you have something to fix the binocs to, move your head away straight back.

If the binocs are out of alignment, the centre images won't overlap. When you look straight 'through', the centre image should be just like looking through a tunnel. If so, congrats, the binoculars are aligned - for you!

Please note that someone else, whose eyes are a different width apart than yours, may still notice an error. This is because the "knock" - and your correction - might not be along the axis of the hinge.