How to determine when you have been cured of gum disease?

You don't cure it: Remember gum disease can be put in remission and kept there with the right oral hygiene and professional cleanings.....But you don't cure it.

Dental examination : After undergoing treatment for periodontal disease, measurements called probing depths are taken by the dentist. These probing depths help determine your stability or if further treatment is indicated.

Radiograph: X ray is the best reference. However, it takes 4-6 months prior to see any bone change on xray. Moreover, your gum disease can also be cured with repair instead of regeneration. In this case, periodontal probing would help to make assessment. Gum should not bleed (or minimally) with tight, flat, light pink color, firm texture. Follow up 6 months with xray and compare radiographs is the best.

No cure, sorry: It is a chronic disease, which we can make better and manage, but it usually doesn't go away forever. The only known "cure" is to pull out the involved teeth, but please don't do that if at all possible.