Archive link: https://archive.ph/cIz4A
It’s dated to be from around 2400 BCE. The article doesn’t clarify if it’s a true alphabet or an abjad, but either way it’s interesting.
EDIT: see also https://www.scientificamerican.com/article/worlds-oldest-alphabet-discovered/ for a less pop-linguistics narrative of the same discovery.
That’s more like a common pattern than a sure way to know it.