Not sure how legal it is but one site I worked at relied on a "two manager" approach, ie two managers had to agree the person seemed like they had been drinking; and the smell of alcohol was enough. Personally it could be tricky ground because if used irresponsibly, it could lead to someone being "got rid of", however, as tribunals only require evidence so that on the balance of probabilities it's likely the person was at fault, you're likely to get away with it. Another thing they had though at the same site, which I think balances this out, is they had a good policy where if you were honest and approached occ health saying you had a drug or alcohol problem, they would support you to get better (on one occasion only) without detriment to your work prospects.
I have, however, seen this through a friend's eyes from the other side. A friend of mine had been out on a stag weekend and on the Monday morning smelt a bit of booze according to his boss. He was sent home and suspended. He was certain however that there was no alcohol in his system despite the heavy weekend and so went to a police station to ask them to breathalise him; they refused but suggested his GP might be able to do a blood test. As it happens the GP was next door so he got a blood test done which showed no alcohol. On production of this the workplace reduced the penalty from dismissal to final written warning which he accepted just to be able to stay in a job (they were really after him for some reason.) He was later sacked for a minor offence which on top of the warning pushed it to dismissal. Now, because of the GP evidence, the workplace did a "left through mutual agreement" in the end (ie had to pay out some cash to stop him taking it to tribunal). Personally, if I were him I wouldn't have accepted the warning and fought them all the way but I'm more feisty.
So I think you have to be careful and also remember that in the workplace unless you have consulted lawyers and have work which would be dangerous to do after a drink it could be dangerous ground legally to start breathalising. After all you have no legal right to check an employee's pockets!
You do need a policy but that's one for your HR department to draw up with support from unions and lawyers. It's tricky ground. Also personally in the first company I spoke about, I was often managing alone so didn't have the chance to get someone else to verify the drinking thing but I did have someone working for me once who smelled of drink. I mentioned it to him and suggested he was careful and then never smelled it again. Either he was more careful or switched to vodka...
