Ralph began to seal his fate when he was gambling and winning big and didn't want to leave the table and greet Tony, and then refuses a drink with Tony.

then he disrespected the Bing by killing Tracee outside.

His anger at being passed over for a captain position and his resentment of Gigi, who Tony thought was doing a good job, didn't help him either.

Tony tested people by questioning them and when he suspected someone you could see it in his eyes. Gandolfini did a terrific acting job in this respect.