A running shoe should feel secure through the heel and middle of the foot, with enough room for your toes to spread. Try shoes late in the day or after a walk, when feet may be slightly larger. Wear the socks you usually run in, and fasten the laces as you would for a workout.
Stand up and check the space in front of your longest toe. A little breathing room helps prevent the toe from repeatedly hitting the front on descents or longer efforts. Your toes should not feel squeezed together, and the upper should not press uncomfortably across the top of your foot. A thumb-width is a useful starting point, not a rule for every foot.
Walk, jog, and turn in the shoes if the store or return policy allows. Notice whether your heel lifts, your arch feels pinched, or your foot slides sideways. Lacing changes may solve mild movement, but they cannot fix a shoe that is fundamentally too narrow or the wrong shape.
Fit is personal and can differ between models, even from one maker. Compare shoes by how they feel on your feet rather than relying only on your usual size. If discomfort persists, try another shape or seek advice from a qualified footwear professional.