The cleverest of criminals leave behind clues to their crime.
It is so true that even the cleverest of criminals leave behind clues to their plan. See how hard John Garrideb tried to work things out and made such a fool proof plan that anybody would have fallen to it. Yet Holmes was able to sort it out with his applied intelligence. He looked for loose ends and caught Garrideb at various points where he got assured of his fake identity and the loop holes in his plans were visible. First when he pretended to be a American, though he was actually, he was caught due to his British look. Then, when Holmes pointed out Dr. Starr, it was clear that Garrideb had not done his homework that he responded with such conviction as if there was a Dr. Starr in real. Even the advertisement he got published was not edited properly and the language in it was easily recognised to be American English. Thus, though Killer Evans had a well knit plan, he could not fool Holmes.