How do I tell if a vertex has been clipped or not?
You can use the OpenGL Feedback feature to determine if a vertex will be clipped or not. After you're in Feedback mode, simply send the vertex in question as a GL_POINTS primitive. Then switch back to GL_RENDER mode and check the size of the Feedback buffer. A size of zero indicates a clipped vertex.
Typically, OpenGL implementations don't provide a fast feedback mechanism. It might be faster to perform the clip test manually. To do so, construct six plane equations corresponding to the clip-coordinate view volume and transform them into object space by the current ModelView matrix. A point is clipped if it violates any of the six plane equations.
Here's a GLUT example that shows how to calculate the object-space view-volume planes and clip test bounding boxes against them.
Here is a tutorial titled Frustum Culling in OpenGL.