When Jesse spotted Newt at the jewelry store he figure it wasn't because the guy was looking to get himself a diamond stud earring. It didn't take a huge leap of logic to realize that Newt was getting something for Kate. He thought about just letting him be but then realized that maybe if both of them were there he could provide some camouflage of sorts.

"So," Jesse said, moving over to the ring section, which was apparently where Newt was shopping. Very interesting. "I figure if we're both here then no one can just assume that you're here shopping for Kate and accidentally ruin whatever surprise you have planned."