How can I tell if I already had flu? I had stomach ache/nausea for a few days & threw up 5x.Had chills & weakness/sick feeling but no other symptoms.

You Cant. The influenza virus typically causes fever, sore throat, cough and myalgias. It certainly could have caused your symptoms. The prevalance of the flu is increasing in the United States right now and if you were exposed to someone who had the flu, i would assume that you also had the flu. However, if you didn't have a fever, the likelihood that you had the flu is lower.