[lttlrck]

A high fever.

The difference is quite significant and a doctor should know the difference.

I was 40 the first (and only) time I caught flu. I went to the doctor. I don't go to the doctor for a bad cold.

Maybe there are light flus. I don't know - but I don't want the real thing again.

[Johnny555]

while high fever and other symptoms is a rule of thumb, there's still enough overlap between flu and cold symptoms (especially a mild flu vs a bad cold) that your doctor is not going to tell you you have one or the other without doing a flu test.