I don't know your plans for the chicks. If you keep them until they feather out and grow, as Mahonri said, you will be able to tell. There is a way to look at the vent and tell, but even professionals are not perfect trying that.
While one rooster should be enough for 12 hens, don't be too quick to assume you can't keep two. There will be dominance issues for sure, but normally one will be dominant and have his choice of the hens but the other becomes his able assistant, will get his share of the hens, and the two will form a team to protect the flock. You'll have to be careful how you introduce them, but it usually can be done. An advantage of having two roosters is that you have a spare if something bad happens.
Occasionally, if the two roosters are very close in spirit, they will both reject the assistant position and will fight to the death to be dominant. I would not expect that to happen with Orpingtons, but you would have to be prepared for that eventuality. Don't know if you want to risk it.
Another potential problem is, if the hen to rooster ratio gets too low, the roosters can actually damage the hens if they mate too much, especially their favorite hens. With Orpingtons, 2 roosters and 12 hens is probably somewhere in that may or may not happen range. I'm confident you will find people on here that have been on both sides of that.
Thanks
