How do I tell whether the fine cracks on my topsides are crazing or the start of osmosis?

Location is your biggest clue here. Osmosis is a below-waterline phenomenon driven by long-term water immersion, so a network of fine spidery cracks up on the topsides, well clear of the waterline, is far more likely to be gelcoat crazing than osmosis. Crazing is surface cracking in the gelcoat itself, usually from stress, impact, flexing or an over-thick gelcoat, and it stays dry and sits on the surface. Osmosis instead shows as fluid-filled blisters below the waterline that often weep an acidic-smelling liquid when opened. If in doubt, check whether the marks are dry cracks or raised blisters, and where they sit relative to the waterline. Should you find genuine blistering below the water, that is worth investigating properly rather than treating as cosmetic.