There are many different opinions on what constitutes a full head. I think "full head" is a poor description. The issue is whether the coin has a solid strike or a soft strike. There are several complicating factors:

Type

Die polishing. Al three mints tortured their dies. like this coin, die polishing lines abound in the series, and are visually unappealing.

Die state. Badly worn or mismatched dies abound in the series.

Whizzed coins, attempting to make sliders pass as UNCs
Here is what I look for. This list isn't original. I stole it years ago, modified it, and have no idea where it originally came from.
Head:

All three leaves are distinct and complete

Hairline is complete, and distinct from forehead and face, all the way to the jaw

Ear hole or ear slit is visible
Shield:

Inner shield has vertical and horizontal lines

Outer shield has all rivets
Exception: Abraded shield polished die varieties generally lack
both attributes
Hand:

Liberty's right hand is complete
Gown:

Fold over Liberty's left leg (our visual right) is complete

Bottom of gown below Liberty's left leg has all four buttons visible
Feet:

All toes visible on the right foot
Gates:

Substantial detail exists in the wooden gates
Reverse:

Eagle's wings have several distinct feathers on the edge of the wing. The eagle's right wing (our visual left) is the key problem area.

Eagle has visible breast feathers. Very few have more than a few breast feathers.
Very few coins have truly full strikes. The goal is to find strong strikes. They exist for several dates, but are almost nonexistent for other dates.