the definition of intoxication is of less a concern to airman than the effects of carrying a person in breach of the regulation.
Unless you have a disability or illness that would explain slurred speech, unsteady on feet, loudness, aggressive, lacking coordination, then you are likely to be intoxicated.
an experienced person should be able to use their skills to determine the difference between someone with a disability and someone that is pissed. An experienced person uses discretion to make this determination so as to avoid causing offence to the person in question.
The law sadly puts the onus on the crew - tech and cabin to ensure as the final line of defence that the CAR's are complied with.
It has been my experience to err on the side of caution and when in doubt perhaps get a second opinion. At the end of the day it is the decision of the PIC if a person is carried.
When i have requested a PIC to offload or deny boarding over my years as crew i have always in every case been supported by the PIC.
in over 20 years as cabin crew I have only needed to deny boarding on 3-4 occasions. In most cases someone that has had a couple of drinks isn't necessarily intoxicated and the PIC will always rely on the support of the Cabin crew and likewise to ensure the security of pax and aircraft.
Sometimes we get it wrong and we miss them at the door and experience is the difference between managing the situation and ensuring all pax and crew travel comfortably and safely or overreacting and cutting of service unnecessarily.
the quantity of alcohol is not always the issue its often the ability of the person to handle it and sometimes its a judgement call on the part of the crew