How do you know if you have the flu or a common cold?

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I've heard doctors speak of the fevers and body-aches and whatnot, but how does one really know if they have the flu or just a cold? I haven't received a flu shot in over 6 years because I was convinced they made me sick within a week of taking it. I don't think I've had the flu since then, while I do get a common cold about twice a year. I'm only asking because I've had either a month-long cold or (most likely) bad allergies. I'm sure it's not the flu but it made me think.