My Hobie 20 has what seem to be gelcoat cracks. Is there a way to determine if these are structural or cosmetic? How would one go about repairing these?

Where are the cracks? There was a known issue with a certain year(s) of Hobie 20 hulls forward of the main beam. In some cases it resulted in a serious structural failure.

They are between the forward and rear cross beams. They are around the dagger board trunks and run towards the back of the boat.

Are they on the deck? Or the sides of the hull? If on the deck, do they begin at the trunk?

that doesn't sound too serious but a more detailed description, or better yet, a photo would give us a better idea. The only real critical thing in that area is the point in the hull where the shroud attaches - you shouldn't have any cracks that look like stress marks in that area.