Are there any easy rules on how you can tell if a noun is countable or uncountable? Or do you simply have to memorize them?
If it's sold by the unit it's probably countable. but if it's sold by the gallon or pound it's probably non-countably.
For the most part, you have to memorize them. But in general, think about how you would buy the noun in question at the store. Can you buy it by the individual piece (Apples, oranges, watermelons)? Or is it sold by the cup, the pound, or the liter (Cereal, milk, rice, virtually any kind of liquid)?
Animals are usually countable (pigs and cows), but their meat products (pork and beef) are usually uncountable. Fish is uncountable, and so are most kinds of market fish (tuna, salmon, trout, sea bass).
This is far from absolute--I have no idea why beans are countable when comparable commodities like rice and corn are not, for example--but it's a useful rule of thumb.