Both my boys got croup at least once a year since the were 6 months old (now 7). A few times in hospital for a few days.
You will definitely know as it is a very distinctive cough with a slight weaze and yes they will eventually struggle to breathe. Don't muck around once the breathing becomes difficult as it means the wind pipe is closing up. I am no doctor, and I am happy to be corrected but this is my experience and knowledge. Good luck. :/)