How do I see what a file looks like after preprocessing?

How can I check the results of preprocessing? For example, say I have the following code:

#define CONCATENATE(X, Y) X ## Y
#define STRING_1 First
#define STRING_2 Second
#define STRING_3 CONCATENATE(STRING_1, STRING_2)

Is there a way to make sure STRING_3 will be expanded to FirstSecond later in the program?