Damage is structural as soon as the laminate under the gelcoat has cracked or come away. You recognise it by a hollow sound when tapping, by hairline cracks in a star pattern, and by a spot that gives under pressure. If the damage stays in the gelcoat, it is cosmetic.
The difference in one sentence
Gelcoat is the coloured top layer and carries nothing. The laminate beneath it, glass fibre and resin, carries everything. Damage that stays in the gelcoat is a question of looks. Damage that reaches the laminate is a question of strength.
Four signs that it goes deeper
A hollow or dull sound when you tap across the area with a coin. Sound laminate rings clear and short; loose or wet laminate sounds dull. Always tap a piece of healthy hull as well, so you can hear the difference.
A star pattern of hairline cracks around a point of impact. That means the laminate absorbed the blow by flexing, and that cracking has formed underneath.
A spot that gives when you press or step on it. On a deck that points to delamination or a perished core.
Water in the bilge with no identifiable leak. Then water is running into the laminate somewhere.
Where it matters most
Around the keel bolts, the mast step, the rudder and the skin fittings the forces are greatest. Damage that would be cosmetic on the aft deck is worth a thorough inspection in these places.
The same goes for the area around the rubbing strake on boats that lock through a lot. Repeated impact loading causes fatigue in the laminate there, even without a single obvious blow.
What you can do yourself
Tapping out costs nothing and tells you a lot. Do it around the damage and half a metre beyond, because loose laminate almost always runs further than what you can see.
Take photos close up and from a distance before you touch anything. If it does turn out to be structural, the insurer needs exactly those shots.