Sadly there is no easy way. You just have to learn the uncountable ones. You can find a list of them online.

Can you count milk? You can't say one milk, two milks, right? So milk is uncountable. You can say one glass of milk, two glasses of milk... but here you count the glasses not the milk. Milk is therefore still uncountable. You can, however, count books. You can say one book, two books, etc. Book is a countable noun. Looking at nouns that can be both uncountable and countable. Water is an example. To determine whether a noun is countable or uncountable, look at how it is being used or what meaning is being applied. For this, you may have to refer to a dictionary to figure out the different meanings. Water as a substance (Eg: there is water on the floor) is uncountable. You can't say there is one water on the floor. You can say there is 1 litre of water, but again you're counting the litres, not the water. So water is uncountable in this context. But a second meaning of water is "an area of water" (Cambridge dictionary). So a sea, lake, ocean, etc. Now seas, oceans, lakes can be counted. So water is countable in this context.

When you are unsure about it, you can look up dictionaries like Cambridge or Collins.