Listen to them. I know it sounds weird but by just holding them and listening to them you will be Abel to tell. Usually you can hear a tiny heart beat or you can feel by their weight.

I have 7 duck eggs in an incubator, it is the 32nd day passed their due date! One egg has hatched and the duck is healthy, he hatched 2 days ago.. Are the other eggs dead? None of the eggs showed any sign of movement through the whole month not even the one that hatched so i dont know if they're dead or not? What do you think?