The whole green top falls over to lay on the ground. The neck between the bulb and the tops turns soft, squishy and then begins to dry. That causes the tops to lay over. Then you can pick if you wish or you can leave them until the neck is dry enough that when you pull on the top it all just comes off in your hand leaving the onion in the ground. ;)

Dave

The thing is, when they bloom it makes the onions more prone to neck rot. The ones that flowered be sure to eat first, as they wont store as long...Onion sets are baby onions that are tricked into thinking they are in their second growing year and need to produce seeds (Biennial of however u spell it)
Usually they dont fall over until way late in the year...And they dont just fall to the ground, they wither from the top down. When they say the tops turn brown and start withering down. You can pull them out and leave to cure in the sun until all dry.
That being said, Being in almost the same zone as you, its WAY too early to pull them up...Even if one or two leaves are withering down. That can be from damage to the leaves from the wind or being knocked over...I would do a test pull if you still arent sure, and check the size. If they are big enough for you then allow them to sun cure then braid them or keep them dry and in a airy place to insure you wont be helping the rot along...
For future reference when you buy sets, make sure you use the TINIEST onions in the pack. The TINIEST onion sets make the biggest healthiest onions that dont bolt and then you wont have to worry about getting neck rot, and youll have onions long into the winter.
I used to use the biggest sets, thinking bigger is better, and always ended up with rotten onions from neck rot... I learned to use Onion starts, not sets, and used onion the smallest sets sent, and used the bigger ones for green onions. -Cassie