There really is no objective way to measure it. That is not to say that above a certain level, one is probably going to be drunk.
But one can become drunk merely from believing that one is drinking alcohol -- a large part of the drunken experience is based on one's expectations of becoming drunk.
This article describes what I'm talking about
:
http://news.bbc.co.uk/1/hi/sci/tech/3035442.stm
I personally do the following to see if I've had too much to drink: I close my eyes and swing my head around in figure-eights for about ten seconds. Then I try to focus on a point in the distance. If it takes longer then a few seconds, I stop drinking. It's far from a perfect test but it's easy to do. Note that this is not a test to see if I'm safe to drive but a test to see if I can drink more without getting sick.