How did sherlock holmes first know that Spaulding was indeed the notorious thief.

Holmes realized from the beginning that the Red-Headed League was simply too preposterous to be real and that it must therefore have been a ploy to get Wilson out of his shop for a few hours every day. The fact that Spaulding was willing to work for so little money and spent a lot of time alone in the basement suggested to Holmes that Spaulding was doing something illicit in the cellar. When he noticed the bank nearby, Holmes had suspected that Spaulding was digging a tunnel to the bank. Holmes pounded on the sidewalk outside Wilson's shop to determine whether the ground was hollow underneath, and he knocked on the door for directions so that he could see whether the knees of Spaulding's pants were worn away. The fact that the league dissolved so suddenly suggested to Holmes that the robbery was imminent, and he was therefore able to make preparations and capture John Clay.