[Connor]

I have put some eggs in incubator to check if the rooster is firing blanks and was wondering is it ok to keep the eggs in for 7 days then take them back out and see if they are developing?

[HesterF]

Have you a candler? Do you mean just take them out briefly to have a look with that or crack them open and have a look? With a candler you can certainly see after seven days whether they're developing or not, with the advantage you can then keep the viable ones going by just popping them back in the incubator and take out the duds. Clearly cracking them open tells you whether he's fertile or not but doesn't help you hatch any!

[Marches Farmer]

Take them into a dark room and hold a powerful torch behind them - if they're fertile you should be able to see a network of blood vessels radiating from a central point.  If the cockerel is young and has been healthy all his life then you've probably nothing to worry about.