Classic signs include labored breathing, abnormal breathing sounds and sometimes bubbles from the nostrils....
but even a lethargic snake can have pneumonia.....ultimately x-rays with your vet can confirm your suspicion ....as well as a white blood cell count...
in the meantime you should increase the temp. in the enclosure by up to 5F .......quarantine the animal and monitor the rest
Dr. D. Auger DVM
c/o Blair Animal Hospital
849 Shefford Rd
Ottawa Ont. Canada