Determine if RRD files are "full"

Does anyone have a script (or even most preferably a nagios plugin) that 
can determine if an RRD file is "full" and losing data? I believe I have 
some RRD files that were created many years ago that are now averaging 
data off the back-end and I'm essentially "losing" data because I didn't 
create the RRD with enough rows to begin with. I need a way to determine 
which RRD files are in this state so I can attempt to increase the size 
of them and keep a longer history.

Thanks!

Eric.