There are several ways to check for new features provided by the Cocoa frameworks at runtime. One is to look for a given new class or method dynamically, and not use it if not there. Another is to use the global variable NSAppKitVersionNumber (or, in Foundation, NSFoundationVersionNumber):
double NSAppKitVersionNumber;
#define NSAppKitVersionNumber10_0 577
#define NSAppKitVersionNumber10_1 620
#define NSAppKitVersionNumber10_2 663
#define NSAppKitVersionNumber10_3 743