enum {
	gestaltSysArchitecture = 'sysa',   /* Native System Architecture */
	gestalt68k	= 1,	               /* Motorola MC68k architecture */
	gestaltPowerPC	= 2                /* IBM PowerPC architecture */
};

A snippet of code that uses this could look something like this in your 68K program:

#include <gestalt.h>
#include <stdio.h>
#include <Types.h>
 
    long myattr;
    OSErr err;
 
 	err = Gestalt( gestaltSysArchitecture, &myattr;);
 	if (err == noErr) {
		if (myattr == gestaltPowerPC) {
			InstallExtraPPCComp();
		}
	} else {
		// handle error condition
	}