How do I check if a decal has loaded?

The PreloadAsync function loads multiple decals at once rather then the old Preload which in tern only loaded the decals 1 by 1

I've re-written the code from the PreloadAsync page from the page you specified, Hopefully its a little clearer on how to use the new Async preload function.

Note: the table below in the code isn't required to use PreloadAsync function its just a way to set a flag (true/false) when the decal has loaded or not you can use a simple table like:

local ContentProvider = game:GetService("ContentProvider")
local Decals = {
	"rbxassetid://1234561",
	"rbxassetid://1234562"
}

local function getStatus(contentId, status)
	if(status == Enum.AssetFetchStatus.Success) then
		print(contentId.." Loaded!")
	end
end
     
ContentProvider:PreloadAsync(Decals , getStatus)

Might be a better format:

local ContentProvider = game:GetService("ContentProvider")
local Decals = {
	{
		DecalID = "rbxassetid://1234561",
		Loaded = false
	},
	{
		DecalID = "rbxassetid://1234565",
		Loaded = false
	}
}
local DecTble = {}
for k,v in pairs(Decals) do
	DecTble[k] = v.DecalID
end

local function getStatus(contentId, status,Table)
	if(status == Enum.AssetFetchStatus.Success) then
		Table.Loaded = true
	end
end
     
for k,v in pairs(Decals) do
	ContentProvider:PreloadAsync(DecTble, function(contentId, status) getStatus(contentId, status,v) end)
end

for k,v in pairs(Decals) do
	print("[",k,"] Decal = [",v.DecalID,"] Loaded = [",v.Loaded,"]")
end

Hope this helps! :)