The answer to this depends on your gas company, and how they operate. Oftentimes receipt of a Division Order is the signal. Other times you might not know until you receive your first royalty check.
Course you can always trek to your county courthouse to check up on your gas company, or just telephone them from time to time, or write to them, making inquiry about your status.
Finally, talking with your neighbors never hurts. Word gets around sometimes when units are declared, even though not everyone necessarily finds out at the same time.
If you are in Ohio a good way to keep track of drilling activity in your area is with the ODNR oil and gas well viewer:
I wondered about this too. I signed with Rice and they are putting a new pad in maybe 100 yards from my property on the other side of the road. I have not heard anything from the company since I signed last June. I was thinking since there are so many wells near me surely I will be included at some point. I sure hope so!