It's never as easy as you hope. Summed up, I'll need to generate a DC and a AC signal. A few measurements would need to be performed. Hal you're correct, it's in part 68. Here's a quote for those interested. Thanks for your help guys.

In order to determine the REN, for North America, four different calculations are used:
• 25MΩ divided by the minimum measured on-hook DC resistance for an applied voltage of up to 100 volts.
• 150KΩ divided by the minimum measured on-hook DC resistance for applied voltage of up to 200 volts.
• 0.6mA divided into the maximum DC current drawn during the AC on-hook impedance test.
• 8KΩ divided by the minimum AC impedance measured while performing the AC on-hook impedance test (for "B" type ringer).
The largest of these four numbers is listed on the label as the REN of the device. If the AC impedance yields the highest REN (as it usually
does), it is followed by a letter corresponding to the type of ring signal applied during testing. For example, if the highest REN calculated was
0.8, and the device was tested as a B type ringer, the REN would be 0.8B.