FindAssetIds.lua

local HttpService = game:GetService("HttpService")
local API = HttpService:JSONDecode(HttpService:GetAsync("http://anaminus.github.io/rbx/json/api/latest.json"))

local ValidClasses = {}
for _, Description in pairs(API) do
	if Description.type == "Property" and Description.ValueType == "Content" then
		local Properties = ValidClasses[Description.Class] or {}
		table.insert(Properties, Description.Name)
		ValidClasses[Description.Class] = Properties
	end
end

-- for ClassName, Properties in pairs(ValidClasses) do
-- 	for _, Property in pairs(Properties) do
-- 		--print("->", Property)
-- 	end
-- end

local Assets = {}
local Instances = {}

local function Recurse(Item)
	for _, Child in pairs(Item:GetChildren()) do
		for Class, Properties in pairs(ValidClasses) do
			if Child:IsA(Class) then
				Instances[Child] = true
				for _, Property in pairs(Properties) do
					print(Child:GetFullName(), Child[Property])
					Assets[Child[Property]] = (Assets[Child[Property]] or 0) + 1
				end
			end
		end

		Recurse(Child)
	end
end

Recurse(game.StarterGui)

for AssetId, Count in pairs(Assets) do
	print(AssetId, Count)
end

local List = {}
for Item, _ in pairs(Instances) do
	table.insert(List, Item)
end
game.Selection:Set(List)