Question: How did Tiana know that Naveen was going to marry Lottie? We don't find out until later that Lottie has to kiss Naveen in order for the spell to be broken and sure Lottie was attracted to Naveen but still, marriage wasn't mentioned so how did Tiana know? Did I miss something?
Answer: Mama Odie tells Tiana and Naveen that Naveen must kiss a true princess in order to break the spell. Lottie is the princess of the Mardi gras parade so that would make her a princess and that would make it necessary for her to kiss Naveen and break the spell.
Tiana knew that Naveen was going to marry Lottie because Naveen tells her when they are running away from danger in the bayou. Naveen says 'when I marry Miss Charlotte LaBouff...' in an attempt to explain to Tiana how he intends to be rich again.
Naveen wasn't looking to marry a princess, just a "little honey whose daddy got dough", as said by the Shadow Man.
Answer: Naveen told Tiana about his plan to marry Lottie once they arrived in the swamp. After revealing that he's been cut off by his parents, he explains that he plans to regain his wealth by marrying her so that he'll inherit her father's prosperity.