How long should I wait?

So I took some eggs from broody mama and I am unsure how long I should try incubating them. There were 2 nests and I put all the eggs together in one nest until I found a dead baby. I took all eggs from them in an attempt to incubate them the rest of the way but I am unsure how long the mamas were sitting. So should I wait about a week? Should I try to crack one open to see whats up? What should I do? I have them at a steady temp in my throw together incubator but I am just so unsure. Please. Any help would be greatful. I am at a point where I want to crack one open to see whats going on in there. Not sure if this is a good idea or not though. Thanks for everyones help. Everything is appreciated!