If it's a standard PC, laptop or netbook based on a mainstream processor (such as an Intel Pentium, Core or Atom or an AMD Athlon or Phenom family CPU) then it's little-endian. If you're running Windows on it, it's probably little-endian. If you're not running Windows, you probably already know exactly what processor you've got and its endianness!
If I remember correctly, the old Motorola 68000 family of processors, used in the early Apple Macs along with the Amiga and Atari home computers were big-endian.
Other types of processor such as ARM (very common in smartphones and tablets), IBM POWER, Sun SPARC and Intel Itanium (mainly seen in high-end servers) can operate in either endianness - they are said to be bi-endian.
Stephen