Hatch assist advice needed. :0(

I have a chick that was shrink wrapped. I need to know how to tell if the membrane is mature enough to open and let the beak out. I know the veins in the membrane have to quit being quite so active at some point can I look at it and tell. I know the baby probably gets oxygen from the blood vessels in the membrane until they quit flowing. I know the quit flowing thing is destined to happen anytime with my chicky. It has a small opening in the membrane ( very small) The hole isn't big enough for it to get enough oxygen once the veins stop flowing. I try to make an opening yesterday for it to breath through because, there was no fluid left. Can I tell when the veins in the membrane have stopped flowing so that I can open a slightly bigger hole in the membrane for it to breath.

Thanks for the help.

 

Thank you, your advice was very helpful. :0) The wetting being able to help me see the veins.