How much of RingIO buffer is used?

How can I tell how much of the RingIO buffer is used?  What I mean by this is, when pushing through fixed-size buffers, how many buffers have been put in by the writer but not yet read by the reader?  The answer could be in bytes or buffer count.  I need to analyze my video system latency, where video frames pass from the DSP to ARM via RingIO.  I need the answer to almost always be 0 or 1 buffer, even though I need a larger ring for random hickups.  I'd rather not rewrite the current watermark code, that's not used for this analysis purpose.