You can connect your telemetry radio to a computer or Android OS device and through your ground control station software ( be it Mission planner or Droid Planner ) you can monitor the number of satellites you're locked on to and the HDOP value you currently have. When the main LED on your IRIS+ turns green, it means you're ready to fly and have achieved GPS lock. By monitoring the HDOP value you can determine how precise flight modes that depend on the GPS will be ( Loiter, Guided, Follow me ). The lower this value, the better.
Note: Make sure the HDOP value is always under 2.