The orientation of a polygon tells us if the vertices are ordered clockwise or counter-clockwise as we move along the perimeter. This is determined by calculating the polygon’s signed area which is positive for counter-clockwise orientation and negative for clockwise orientation. A quicker algorithm is to determine the signed area of the triangle formed by the lowest right most vertex and it’s immediate neighbors.
The details of the solution can be found in the CGA FAQ linked above. A C++ implementation can be found here: http://code.google.com/p/curve-project/source/browse/trunk/lab/cgafaq/cgafaq.h The function cgafaq::orientation has two overloads. The first computes orientation for three vertices and the second calculates the orientation of a polygon.
Disclaimer: This post and the code provided are not endorsed by any of the contributors to the CGA FAQ.