Flu commonly but not universally comes with fever, while colds only rarely come with fever.
Flu (Influenza): Causes, Symptoms, Types & Treatment contains a chart of symptoms of flu, colds, and COVID-19. However, a set of symptoms may be ambiguous (plus there are other things like RSV that can give similar symptoms), so testing may be the only way to find out for sure (but at home tests are limited to flu and COVID-19 in the US).