Determining how similar a signal is to a sine wave
Assuming that the description I made of the FreezeFramer process is correct, then I would try determining the SINAD rating of the peak frequency in the interpolated data.

You have a sampling rate of 2Hz, giving a Nyquist of 1Hz.  You will be looking for a sinusoid with a frequency well below 1Hz.  The SINAD value is (basically) the amplitude of the peak frequency divided by the sum of all the other frequency amplitudes.  There are better descriptions available, as well as implementations for many of the more common DSP frameworks.