To hide that he is a warlock from the players meta knowledge, do not describe his abilities with meta terms. When he casts an Eldritch Blast, don't say Eldritch Blast, but instead describe that he invokes some magic.
An Arcana check would then be a good tool for them to find out more about that magic ingame. But you already know that one.
From there you only need to give them hints and opportunities to clue them in that they are a Warlock.
They could see him reading in a book which he then quickly hides, if they are interested they could try to get their hands on that book.
They could see him in a Situation where he has to use his Warlockpowers, and could then use the Arcana check to see if they know something about the magic.
No other specific mechanic comes to mind, and as other answers have mentioned the players wont be able to find out that he is of the Warlock class (because that doesn't exist ingame).
But they can find out that he is what people in your world refer to as a Warlock by observering his actions in your world, and comparing that to what people in your world generally refer to as a warlock.
In most worlds that would be stuff like:
He reads forbidden grimoires, he sacrifices life to gain power, and so on.
But in your world it could be whatever NPCs say it is:
Maybe you notice Warlocks by a Marking they have somewhere on their body, maybe they sleep during the day, maybe they don't like Brocoli.