VNC doesn't really have a concept of frames. The RFB protocol sends
"framebuffer updates", which can encompass anywhere from 1 pixel to the
entire screen, so the best that the VNC viewer could do is measure
pixels/second or updates/second or, from the network's point of view,
bits/second. In some cases, such as playing a movie, updates/second
might correspond to frames/second, but there is no way for the viewer to
know that. If you are trying to measure video playback, then you might
be able to use TCBench, which is a program provided in the VirtualGL
source tree. It latches onto an existing window and monitors a small
region of pixels (by default in the center of the window, but this is
configurable) and reports how often those pixels change. That can give
you a rough estimate of frame rate, as seen by the viewer.