How can you tell if pneumonia has developed from a flu ?

Cough, fever,chills: Yor will know because you are not better. A flu may last 3 to 5 days and then you get better. Pneumonia, you will not get better, you may have chills, fever, nausea vomiting, unable to breath, you may have a persistant cough and fever. Generally, you will need plenty of rest. You could get walking pneumonia, but you will still feel miserable. A cxr will tell the tale.