I normally buy paneer only if it's packed within previous day of two. Apart from checking the date of packaging, is there anyway to check whether the paneer is fresh or sour? Also, when paneer itself has a tinge of sour taste, why do people soak paneer in vinegar before using it in curries?
Paneer is a fresh cheese, which gives it a short shelf life, typically under two weeks if kept refridgerated. Outside we are talking a few hours. The higher water content of soft cheeses makes them a better medium for the growth of pathogens.
Paneer will go bad before one can tell from visual inspection. If you are like 90% of the population, trying a small piece will not harm you. If it tastes like sour milk, toss it.
Using vinegar is an old habit from times when refrigeration was not common. On foods with low moisture content, such as dried meats, a small spot of mold may have been washed with vinegar (an acid) preventing it from growing further. That idea must have transfered to paneer, but it is not applicable there because its water content makes it likely that the pathogens are also inside.