I have had many sexual partners and now I am worried that I may have caught HIV as I keep getting colds and flu all the time. How do you know if you have HIV?

The only way to know for sure is to have a blood test. The fact that you are worried about it now is a good enough reason to get checked out. Remember that HIV can take years before it shows any signs or symptoms. However you may be run down for another reason. It would be good to go along to a Family Planning Clinic, Sexual Health Clinic or your own doctor for a sexual health check up.