What was the single most important clue which gave Holmes an idea of Spaulding's motives?

The single most important clue that gave Sherlock Holmes an idea of the motives of the man who calls himself Vincent Spaulding in "The Red-Headed League" is not one of the several clues the detective picks up from listening to the tale of Jabez Wilson. These clues tell Holmes that Spaulding is up to something, but he has to go to Wilson's pawnshop and examine the neighborhood before he understands what this assistant, whose real name is John Clay, is up to. While Holmes and Watson are in Saxe-Coburg Square, the detective knocks at the door of the pawnshop and inquires the way to the Strand of the assistant. After the case has been solved and both Clay and his accomplice have been taken off to jail, Holmes explains his line of reasoning to Watson, including this:

I surprised you by beating upon the pavement with my stick. I was ascertaining whether the cellar stretched out in front or behind. It was not in front. Then I rang the bell, and, as I hoped, the assistant answered it. We have had some skirmishes, but we had never set eyes upon each other before. I hardly looked at his face. His knees were what I wished to see. You must yourself have remarked how worn, wrinkled, and stained they were. They spoke of those hours of burrowing. The only remaining point was what they were burrowing for. I walked round the corner, saw the City and Suburban Bank abutted on our friend's premises, and felt that I had solved my problem. 

The condition of the knees of Clay's trousers tells Holmes that Clay is digging a tunnel. By beating on the pavement with his walking stick, Holmes ascertains the direction in which Clay is digging. It can only be towards the City and Suburban Bank. Holmes makes inquiries and learns that a huge amount of French gold coins is currently stored in the underground strongroom of the bank. The knees of Clay's trousers are the most important clue because they tell Holmes exactly what Clay has been up to. They explain the crook's creation of the Red-Headed League and the object of all his machinations since he came to work for Jabez Wilson at half-salary. Holmes is able to set a trap which nets both Clay and his accomplice, and the detective saves the bank from the loss of a fortune in gold coins which the two burglars intended to loot from the bank's strongroom on Saturday night, the night of the same day they had posted a notice on the office-door announcing that the Red-Headed League had been dissolved.