Assuming you still have the original plans, measure an obvious dimension - the length of the hull would be good. Multiply this by 24 to get the original length. Measure the same dimension on the new plan and divide this into the original length. This will give you the new scale.
Example:
Plan length 20 inches
Original length 480 inches (20 x 24)
New length 16 inches
New scale 1/30 (480 divided by 16)
Of course Murphy's law says your new scale won't be a round number

HTH
Doug