You recognise osmosis by blisters on the underwater hull, visible as soon as the boat is out of the water. Break one open and moisture comes out with a sour, vinegary smell. Only a moisture reading gives certainty: blisters can also form because antifouling is not adhering well.
What osmosis actually is
Gelcoat is watertight but not completely water-repellent. Over the years water molecules work through the gelcoat and react with soluble substances left behind in the laminate. The reaction product attracts more water, the pressure builds, and the gelcoat is pushed up from the inside. That is the blister you see.
So osmosis is not damage from the outside but a process taking place inside the laminate. That explains why sanding back and respraying solves nothing: the problem sits under the layer you are taking off.
Wicking or structural osmosis
Wicking is the mild form: very small blisters, usually concentrated around the waterline. Many boats of twenty years or older have it to some degree, and they sail on with it without trouble.
Structural osmosis shows as larger blisters, roughly five to fifty millimetres, spread across the whole underwater hull. Here the laminate itself is affected and it does not go away by itself.
The distinction decides whether treatment makes sense. It is also exactly the distinction that cannot always be made from a photo, and for which somebody has to come by with a moisture meter.
Judging it yourself: what to look for
Look straight after the boat comes out of the water. After a few days ashore small blisters dry up and are much harder to see.
Press gently on a blister. If it gives and moisture comes out when it breaks that smells sour or vinegary, it is osmosis. If it smells of nothing and there is air inside, it is probably an antifouling adhesion problem - a completely different and much smaller issue.
If in doubt, have a moisture reading taken across the whole underwater hull before any money goes into a treatment. A treatment based on the wrong diagnosis is an expensive mistake.
Treat it or leave it
Osmosis is rarely an immediate danger. A boat with mild wicking can sail on for years without anything going wrong. But it never clears up by itself, and every season the treatment gets bigger.
The honest trade-off is about the age of the boat, how bad the damage is and how long you want to keep it. On a boat you will sell in three years, a full treatment rarely pays for itself. On a boat you will sail for another fifteen, putting it off costs more.
An osmosis treatment is calculated per square metre of underwater hull. Allow for a winter season of lead time: the drying alone takes weeks and cannot be hurried. What it will be in your case, you will hear after the moisture reading.