I think the only way to tell if the message was sent is when the lights stop flashing. As long as they are flashing, it's still trying to communicate with the satellite and will do this for I believe -13 days continuously.
I look at this like a beacon, it's not a guarantee that you're going to be rescued/found but at least I'm adding some insurance.
You are correct, if 2 lights are flashing, it is still talking, when only the ON light is flashing, message was Sent OK,
From Spot:
Functional Specifications
SPOT features four key functions that enable users to send messages to friends, family or emergency responders, based upon varying levels of need:
Alert 9-1-1 – Dispatch emergency responders to your exact location, SPOT sends one message every 5 minutes until power is depleted or 911 is canceled.
Ask for Help – Request help from friends and family in your exact location, SPOT sends one message every 5 minutes for one hour or until Help is canceled
Check In – Let contacts know where you are and that you’re Okay, SPOT sends three identical messages to the SPOT service for redundancy. The first of those three messages is delivered.
Track Progress – Send and save your location and allow contacts to track your progress using Google Maps™ SPOT sends one message every 10 minutes for 24 hours or until SPOT is powered off.
Battery Life:
Includes 2 AA 1.5V lithium batteries; under normal usage a full battery charge should meet or exceed the following
* Power on, unused: Approx. 1 year
* SPOT casting tracking mode: Approx. 14 days
* 9-1-1 mode: Up to 7 consecutive days
* SPOT check OK/√: 1900 messages