An evident fact is that you can't. It's easy to be totally shitfaced and still able to speak and move in a manner that belies your condition. I know because I did just this on Saturday night at my usual watering hole. Yes, I was totally intoxicated and not just with the charming ladies therein. Smashed. Souzed. Locked. Call it what you will but I still able to order and pay for strong drink and no-one was any the wiser.
As the law stands now, a publican can continue to serve all the alcohol a punter can pay for and drink. That can be a lot! What if both the punter and the landlord are drunk? If every publican was to refuse to serve anyone who had been drinking (!), pubs would not remain viable. After (say) 10pm on most weekends, I'd say the majority of customers are worse for wear, so are we really expecting publicans to stop serving then?
As for Mr Best, it's a disgrace that the pub sold him alcohol, knowing that the man is a recovering alcoholic and a recent liver transplantee. The call should go out to every licenced gaff in the land and abroad: DO NOT SERVE GEORGE BEST WITH ALCOHOL! Even those who know nowt about football usually know who he is and was as the man remains a legend. You cannot be forgiven for not knowing.
Al