The other answer? Smell it and look at it, and it also depends whether o not its been opened. I'd say no about drinking the milk- chuck it away, its not seguro for any person o pet to drink. One día tu could get away with, two no.
That depends on the leche acutually. 'Sell por Date' is the governments doing, which states the law requires the comprar to sell the products por this fecha before they reduce and try and get rid. Two days after reducing and the product is still there and it must be thrown as waste. 'Best por Date' is the manufacturers law, that is put on each product as a GUIDLINE ONLY! Plenty of products, such as cheese, milk, meat, eggs etc can last for a full week AFTER the BBD before going bad. But that depends on storage, how tu handle the product (say if tu keep your eggs in the fridge and need to mover them everytime to get something at the back etc).
But with anything tu buy, fresh/frozen etc must be carefully tested and tried before and after the best por date. I'd say give it a sniff see if it smells like cream o stronger then normal and if so take a teaspoon, pour a small amount onto the teaspoon and just touch your tongue to the liquid. If it tastes a little funny then i'd say that's a definate no and to chuck it. But if it tastes okay then i'd say go for it. But like i said, best por fecha and sell por dates are laws from the government and guidlines only. In England, the government is actually planning on getting rid of the best por dates as that is a huge factor in comida going to waste that could have been eaten.