Besides being tested for COVID-19, the best way to know if you are infected is to look for symptoms. The earliest symptoms that most often occur include fever, muscle pain, cough, and sore throat. However, symptoms can occur any time between 2-14 days after infection, if they appear at all.
References:
- CDC: https://wwwnc.cdc.gov/eid/article/26/6/20-0301_article#:~:text=Early%20symptoms%20in%20most%20COVID,infections%20(6).
- Johns Hopkins: https://www.hopkinsmedicine.org/health/conditions-and-diseases/coronavirus/coronavirus-symptoms-frequently-asked-questions
- CDC: https://www.cdc.gov/coronavirus/2019-ncov/symptoms-testing/symptoms.html