He shows up with flowers. He takes you to dinner. He brings you presents.
Does he ask you what kind of flowers you like? If you tell him "yellow roses," does he never bring you anything else? Does he order the meal when you go to that expensive restaurant? Does he seem to know about wines? Is he picky about the meal? Does he complain to the point of embarrassment? Does he insist that you eat exactly the same thing as he does?
Does he start bringing you presents? Maybe a teddy bear? A BIG teddy bear? So big that there's nowhere to put it except in the middle of the room? How about jewelry? Has he started giving you earrings? Necklaces? Rings? Just remember, you will be expected to wear the jewelry he has given you every day. And, don't even think about wearing any other jewelry. He will accuse you of getting it from some other guy. You can explain that you bought it yourself. You can show him your grandmother's Will, to prove that locket came from her. He won't believe it. Some Other Guy must have given it to you. Hence, you are cheating.
He will go through your purse, checking for phone numbers and photos. He will look through the contacts on your cellphone. He will go through your caller ID on your landline. He will attempt to hack into your email. He will check your odometer. He will go through your mail, including phone bills.
I dated a guy like this once. One day, a red car passed us on the freeway. I glanced at it. He pulled over and yelled at me for 15 minutes, wanting to know who the guy in the car was that I was flirting with, and where we had arranged to meet.
You cannot win with these guys. No matter where you are in this cycle, get out, now. It won't get better; it only gets worse.