I have had flu, just the once, and you can definitely tell the difference between that & a bad cold.
I didn't have a proper cold as such when I had the flu. I had a chest infection at the same time, so coughed a lot, but didn't have a head cold at all.
I barely moved from my bed for 10 days, had a raging temperature, suffered terrible night sweats, was very very weak, had no appetite, and have never felt so ill.
I remember taking a bath, and just lying on the bathroom floor for a good hour after, as I felt so ill & weak.
I lost about half a stone in a week, and had to rely totally on other people to take care of my children. It was just the pits of an illness.
I now never confuse a bad cold with flu!