Try on both boots, standing up, with the socks worn for hiking.
Try several models and various sizes if necessary.
Check two comfort points: the heel seating (the heel does not or hardly slips) and the front of the foot on a downward slope (the toes do not touch the front of the boot).
Avoid all pressure points when trying shoes on.
Don't hesitate to walk around the store.
Break in your boots gradually during their first hikes to adapt them to your foot.