What do you mean exactly. If it drops into the available to register pool? .com drops at 2 pm I believe. So anytime after 2 it will drop. You have to go to a site like domaintools or godaddy and keep typing it in until you see it drop. But if a big player is going after it you have no chance.
If you want to check if a domain dropped just go check if it's available or not. Available = Dropped (assuming it was reg'd before). .........