I’m never drunk as long as I can support the floor with my back.
I used to be a very happy drunk. I had a friend in college who would say that “You know Khadaji is drunk because his smile goes from here to here” and he would mime a big wide smile twice as large as my face.
Now it can make me tired and cranky (age, I guess, but maybe other health issues) and so it just depends.
Is the answer: when you’re posting in threads entitled “what are you drinking right now” and discover yourself browsing in the “we have pigeon eggs” thread? 
(it’s just a joke, not intended to be a jab at SiamSam who’s one of my favorite posters)
when I’m trying to make a left and I can’t distinguish the road I’m on from the road I’m turning into to.
Yeah I guess I was a little vague, sorry. Having quite a bit of experience in his area, there is always that period when you think you not drunk yet when in fact you are. I’ve learned to keep track of the number of drinks I’ve had as a guide to how drunk I actually am. There have been too many times when posting here (while drinking) when I ask myself, ‘Is this a drunk post’? After assuring myself it is not and posting, I invariably regret it in the morning.
When totally hammered I get double vision, loony tunes style. God knows what my eyes must look like. Takes some serious bevvying to get to that stage, though.
General drunkeness is indicated by just getting really quiet and withdrawn from any group conversations.
If I can say “I am not inebriated” five times fast, then I am not inebriated enough.
If I’m at a party, I’ll start roaming room-to-room. If I find nothing exciting happening in the the kitchen, onto the living room. Conversation boring in there? Check out the porch. My brain demands amusement like a tyrant, and everyone else is my jester.
Also: “Let’s dance!”
If I get too far gone, my eyelids go squinty and I become really quiet.
I get funnier and louder and more touchy-feely. I’m a happy drunk. 
The numb lips are my first clue that I’m getting a little tipsy.
if i look at a pic of my ex-wife and say whose she? shes kinda cute… then i’m well past drunk.
I start posting on the SDMB.
My ambulatory curves start to go wide, I start thinking my way through activities that I usually don’t have to think my way through, and my alibity to types eem to gredade some how.
When my co-workers seem like a nice bunch of people.
You must be drunk now! 
I get a very obvious flush on my cheeks and my head feels a bit fuzzy.
When I’m drunk, the feeling itself becomes funny to me. So when I laugh about how I feel, I’m drunk.
When I realise my brain -> filter -> mouth order has gotten screwed up, which has on two occasions led to my nose getting somewhat more closely aquainted with the rest of my skull. But usually I hit upon it after having said something hideously off-colour and badly enunciated in public. Like that one Jimmy Carr joke.
If I don’t grasp my inebriation at that point, but rather live to behold the immense mass of my drunken foibles next morning, I can conclude that I was whisky drunk.
Whenever people in police uniforms are screaming and shining bright lights at me, I know I’m probably drunk.
When I can’t feel my face, I know I’m drunk.
When I start flirting with anyone I find even mildly attractive.