FWIW, I asked my doc last week about what steps to take. I said I thought the ER would be the appropriate place only if I thought it was going into pneumonia, and he said I was right. The classic signs of pneumonia are (as said here elsewhere) cough, fever, and shortness of breath.
I've only had pneumonia once in my life, and that was when I was in my 20s (a very long time ago). I didn't know what it was, and just thought it was by far the worst cold I had ever had in my life. Just miserable. Went to the hospital on base where they took a culture and confirmed it was pneumonia. At that point I expected to be given a big bag of drugs, but they just sent me home and told me to rest for a week or two and take plenty of chicken soup.
Seemed like an odd prescription, but it worked exactly as expected. I just laid around listening to music and reading (didn't have a TV then) and getting lots of sleep and chicken soup. Within a week I was fine. Possibly related to the fact that I was young and healthy, but it demonstrated the healing power of the body's own defenses.