Despite what's commonly said, the pecking order often is not in a neat linear sequence (a-b-c).
There can be one hen that is dominant over many/most hens. In a small flock it can happen there indeed is one hen that is dominant over all hens- that is a "true alpha".
However, the pecking order can be and often is like this: Hen A is alpha over B, C, D but hen E whups A's butt. Or each hen is dominant over only a few, and a few others are dominant over her.. this can make a flock seem about "even" with this sort of complicated pattern. This sort of pattern can cause a revolving wheel style around a bowl with yummies..
Dominance of one hen over another can be seen by a hen sometimes pecking at another hen to tell her to get out of the way(as in on a perch) or just away from food/her own personal space. If the hen being pecked suddenly moves away or "swerves" her head, occasionally a submissive hen will raise her wings and softly flap them- this is a submission signal directed at another bird.. you are seeing a dominance pattern.
When a bird is "mad" or wants to warn another bird it thinks is a lesser specimen, she/he slightly drops their wings and tilts tail towards the other bird.. if the other bird does not back off she will turn to directly face the other bird and stand tall and give a few pecks to press the message. Roosters also use this posturing but much more obvious and often add wing and foot stomping action.
Position is settled by fighting(it can either be a few forceful pecks or full blown wing beating and clawing) and once established and stable(no new birds added or removed) it can be so stable there is nothing really obvious for a long time.