As soon as you get in your car whilst drunk you are likely to be committing an offence. Defences to this would be to show you had no intention of driving, you were only doing so in an emergency situation or you were moving the car a minimal of distances.
Evidence that you had drunk more than the legal limit is fairly easy to obtain. There is a roadside breath test followed up by a further test at the police station. Blood or urine samples may also be required.
Driving under the influence of drugs is more problematic for the police, although they are currently working on tests for this.