Take a sliding bevel and align it under the gunnel to your current rod holders. Lock it in position and then match it up to a protractor (your grammar school aged child probably has one in his/her pencil box kit). Even if it is close you will be able to tell if they are 15's or 30's, as these are by far the most popular rod holders.
See the attached link for a sliding bevel or borrow one from a friend in the trades:
http://www.grizzlytools.com/outlet/S...l-7-1-2-/H5888