You can increase the chance of a Chansey with a Lucky Egg appearing by having a Pokemon with the ability Compoundeyes (such as Butterfree) at the front of your party. When the Chansey appears, you can switch from your Butterfree to your Pokemon with Frisk to discover what item it is holding. Shuppet, Banette, and Stantler have the Frisk ability. Hope this helped and Happy Hunting!