It depends. What are you "at risk" for? Learn all you can about that condition. If you're at risk because of a family history, ask family members how they found out they had the condition, and ask them to ask their doctors about screening tests for the condition.
All of them. Sorry, but you need to elaborate as to what you are at risk for to allow an answer. Cancer? Heart disease? Pregnancy? Overexertion? Etc. Need to tell us more.