We know this fact by the times of Rinoa and Squall's births. There's no way Laguna could have impregnated Julia, left Galbadia, become injured, recover from his injuries over a period of months, court Raine, marry Raine, and impregnate Raine in any way for Rinoa and Squall to have the age difference that they do. Caraway is Rinoa's father, not Laguna.