Help! I want to make sure a divice is mobile in my game! how do i do it?

The best way to check if a device is on mobile is to check if the screen can be touched using UserInputService with the TouchEnabled property.

Here's an example of making the Parent object visible when the device is on mobile:

if game:GetService("UserInputService").TouchEnabled then
    script.Parent.Visible = true
end

Edit #1: If you don't want touch screen computers to use this, then also check if KeyboardEnabled is false, like in this example:

local UIS = game:GetService("UserInputService")
if UIS.TouchEnabled and not UIS.KeyboardEnabled then -- If there's a touch screen with no keyboard then....
    script.Parent.Visible = true
end

Edit #2: If you have a server script (not a local script), then I'd use a RemoteFunction. The issues I've had with RemoteFunctions is that you need a certain type of function. This example records what device everyone is using to the server when the player joins:

-- Server (Non-local script)

local isOnMobile = {}
local event = Instance.new("RemoteFunction", game:GetService("ReplicatedStorage"))
event.Name = "MobileData" -- Change event name if you have other events.
game:GetService("Players").PlayerAdded:Connect(function(p)
    local isMobile = event:InvokeClient(p)
    if p and isMobile ~= nil then -- Alot of issues happen with RemoteEvents/RemoteFunctions. Good to check this stuff.
        isOnMobile[p.UserId] = isMobile
    end
end)

-- Client (Local script)
game:GetService("ReplicatedStorage"):WaitForChild("MobileData").OnClientInvoke:Connect(function()
    local isMobile = false
    local UIS = game:GetService("UserInputService")
    if not UIS.KeyboardEnabled and UIS.TouchEnabled then
        isMobile = true
    end
    return isMobile
end)