How can I recognize a Grim Patron Warrior before it's too late?

Solution 1:

The sign to me is usually the presence of an early Unstable Ghoul. Control warriors usually don't play the ghoul, but GPW's do.

Another sign would be the presence is a Warbot. Control warriors don't play this card, but it can be useful in GPW decks. This is usually the earliest, easiest way of telling for me.

Waterseas edit: Acolyte of Pain is a great card that comes out early. This card is used along with Whirlwind, Bouncing Blades or CTM for quick card draw. All of those cards also lead to the drawing of their combo.

Solution 2:

The Grim Patron Warrior (GPW) plays more "Card draw"-cards like Loot Hoarder, Acolyte of Pain (like Control Warrior), Battle Rage and sometimes Gnomish Inventor to get the combo pieces.

Normally they have less "Armor"-cards like Shield Block, Shield Slam or Shieldmaiden.

The GPW I faced also played Dread Corsair.