Heh, well Columbo is naturally suspicious, and has an "eye" for these things, but it's a good question because early on it was subtle. Columbo quickly suspected a murder that involved drugging, because the witnesses heard no scream, so he was looking for who had an opportunity to drug Gabriel soon before the fall. Why suspect Crawford? At first I think he just didn't like Crawford's attitude -- the exaggerated sympathy combined with the drinking and the celebratory "conducting" of recorded music. The guy just seemed a bit off. The earliest specific clue, I'd say, was that Crawford claimed he never saw Gabe alive that day, yet Columbo sees all of Gabe's clothing etc in Crawford's office. He also didn't like that Gabe's house key was missing from the key-ring found in Findlay's office, and it seemed a bit much that Crawford claimed to have no idea where his protege even lived. Not much to go on, but by then Columbo had the scent.