Based on The Detective Card Trick The Card Magic of Edward G Brown by Trevor Hall 1973 Professor Sidney Lawrence also

Before you begin you need a crimped card at position 26 from the top. For impromptu work I would do this Take a shuffled deck and hold it face down. Spread count the cards as you say you're checking that it's a full deck it doesn't need to be . Injog the 26th card as you do this. Finally bend down the inner left corner of the injogged card before pushing it flush 1. Place the deck face down on the table. Point out that it's possible that you could know the top and bottom cards of the deck, so...