Both flu and COVID-19 can cause fever, cough, sore throat, body aches, fatigue, headache, and a runny or blocked nose. The overlap is so large that symptoms alone cannot distinguish them reliably.
Some features were more associated with COVID-19, such as loss of taste or smell, but these are not dependable for diagnosis, especially with newer variants. The only way to confirm which one you have is testing, which can also matter for treatment decisions and for protecting others.
If you are unwell, consider testing where available, rest, and isolate as appropriate to avoid spreading illness. Seek urgent care for breathing difficulty, chest pain, confusion, or bluish lips. People at higher risk should contact a doctor early, as specific antiviral treatments exist for both conditions when started promptly.