Strictly speaking random breath testing is a no no...however.........
You can be stopped for the purposes of checking you are licenced & insured to drive the vehicle you are piloting.
Once stopped, the cop only has to "suspect" you may have had a drink & you're bagged.
You can be stopped for any moving traffic offence & bagged.
Have an accident & you're bagged.
And of course driving in a snake like manner will deffo get you!
The long & short of it is that it is dead easy to justify a breath test which means it's dead easy to get caught.
I personally have witnessed the demise of numerous licences the morning after - one of whom had done nothing wrong but another car ran into him - the offending driver was sober & the "innocent" one was over the limit.
There is a big anti drink drive campaign running nationwide into the New Year....
As for the how much can you drink......
None is the only totally safe amount!