1) Could ask you what you've been up to lately, trying to beat around the bush to see if you're actually dating anybody else. 2) Makes a heavy effort of talking to you or being interested in your life 3) May flatter you, such as liking your pictures or posts very often 4) When they suddenly come out of the blue despite you guys not really talking much before.
First-off, the No Contact Rule is absolute horseshit that will fail 100% of the time. The longer "no contact" is in place by either side, the less probable your failed relationship will recover.
I want to explain why the No Contact Rule fails by giving an example... McDonald's wants you to buy their food so they advertise. You go get a Big Mac and large fries, whatever. One day, some ivory tower "genuis" suit at McDonald's HQ decides to reduce the marketing budget. This person is a firm believer in The No Contact Rule. His thinking is: If we don't advertise, then people will miss us and then want to come back to McDonald's and buy more food. So much for his theory because the reality is that Burger King hasn't stopped advertising - they are not doing no contact and they are keeping in contact with their customer base - so, while McDonald's keeps up The No Contact Rule and doesn't advertise, BK is getting improved sales because those people wanted their fast food but weren't thinking of McDonald's. They were seeing BK advertisements, so they went to BK. After a month, the ivory tower genius suit at McDonald's HQ is fired and his story becomes required learning in business schools across America.
It's the same with relationships. If you have no contact, guess what? The other person is having plenty of contact with your replacement and is very quickly learning that 1) they can survive without you and 2) you really weren't all that special.