Not that I am currently going through what could be a life-altering hangover. But if I were, here are the ways I would know it.
1) If you wake up sometime in the morning (although not sure when, as your clock died weeks ago, which has nothing to do with the hangover and everything to do with the fact that you need to go to the store today, which in turn has EVERYTHING to do with your hangover), and your poor dogs are begging to go outside and get fed, and all you can do is vaguely kick them aside, go to the bathroom, and then crawl unsteadily back into bed.
2) Your roommate feels it necessary to yell from the hallway, "Hey Tassie! Are you alive?"
2a) Because, come to find out, it's 2 in the afternoon and you're still asleep.
2b) No wonder the dogs were unhappy with me.
3) Your roommate laughs and tells you about That Picture of you from the night before. The one with her boyfriend, that many many people on facebook already 'like' and which she is not angry about (one would hope not, as she took it) but instead is jealous of.
3a) You look at the picture, and are unsurprised people find it amusing, as you are wearing rainbow striped thigh highs, heels, jean shorts, and snuggling with your roommate's boyfriend, and really all you see in the picture are legs.
[Steph here using magical admin powers to say that the part of the picture, which is, in fact, amazing that made me cock my head, laugh and let out a 'damn, Tass' wasn't the rainbow stockings, or the jean shorts, or the violet hair, but the heels. This says so very much about both me and Tass :)]
4) You have a vague recollection of blasting Lionel Richie on the record player very very late at night.
5) You finally let the dogs outside, and have to ask your roommate why on earth there's a wine bottle stopper on your back porch.
6) You are every so very thankful that you had some idea where the Advil was.
7) The sound of the clacking on the computer keyboard makes you slightly nauseous.
Not that I'm going through any of that right now. But you know. That's how you WOULD know you were hungover.