Zircon makes some handy little electronic stud finders. I've had one for about 20 years now, it was $10-$20 back then. It finds the ouside edges of the studs/rafters. I mark the edges on a piece of masking tape or a wide Post-It-Note so there are no permanent marks left behind.
Rafters are designed to support distributed loads, say 50 pounds per square foot over the whole roof. Concentrated loads require extra bracing. If adults use the swing the maximum load on your hooks could be between 150 to 250 pounds per hook with a 50 pound side load when it swings. To spread the load between 3 or 5 rafters, consider installing 2x4 or 2x6 braces to the bottom side of your plywood ceiling. Fasten the braces through the plywood into all the rafters with deck screws, dry wall screws are brittle and can snap. Still fasten the hooks into the center of a rafter.