The two overlap a lot, which is why they're easy to confuse, but there are some useful clues.
Onset is one of the biggest tells. A cold usually comes on gradually over a day or two. The flu tends to hit fast, often within a few hours, going from feeling fine to feeling terrible.
Severity is another. Colds are generally mild, mostly a runny or stuffy nose, sneezing, and a sore throat, without knocking you flat. The flu is usually more intense: high fever (often 100-104F), chills, significant body aches, headache, and exhaustion that can make it hard to get out of bed.
Fever is a useful signal too. Adults with a cold rarely run a high fever, if they get one at all it's usually low-grade. A high fever is much more typical of flu.
Both can cause cough, congestion, and sore throat, so those symptoms alone don't tell you much. But the combination of sudden onset, high fever, and severe body aches points toward flu, while a slower build-up of milder upper-respiratory symptoms points toward a cold.
Duration matters as well. Colds typically resolve within 7-10 days. Flu symptoms, especially fatigue, can linger for one to two weeks, and complications like pneumonia are more common with flu, particularly in older adults, young children, and people with underlying health conditions.
If you're not sure and symptoms are severe, came on suddenly, or you're in a higher-risk group, it's worth checking with a doctor. Antiviral medication for flu works best when started within the first 48 hours of symptoms, so timing matters if you think it's the flu.