PowerPC executables store their code in the data fork, while 68k executables store their code in the resource fork. Examine the first 12 bytes of the data fork for the ASCII Joy!peffpwpc to determine the presence of PowerPC executable code. Examine the resource fork for the presence of a CODE resource to determine the presence of m68k executable code. Note that many MacOS executables are "fat binaries" which contain code for both architectures; these can be identified by the presence of a cfrg resource with an ID of 0.
Source: https://support.apple.com/kb/TA21606