WoW:API IsAddOnLoaded

IsAddOnLoaded - Documentation by Flickering

Determine if an AddOn has been loaded.

loaded = IsAddOnLoaded(index or "name")

Parameters

(index or "name")

index: Integer - The index of the AddOn in the user's AddOn list. Note that you cannot access Blizzard-provided AddOns through this mechanism.
name: String - The name of the AddOn to be queries. You can access Blizzard-provided AddOns through this mechanism.

loaded

loaded: Flag - Indicates if the AddOn has been loaded, 1 if it has been, nil if it has not.

if (IsAddOnLoaded("AddonName")) then
  do stuff
end