I'm not seeing these secret signals either. I just figured Crowley set the trap for whomever would be the one to actually bone Dick, and didn't necessarily care if it was Cas or a Winchester. Was there indication that Crowley knew from the beginning that Cas was going to be in the showdown?

Of course this could simply be a gaping plothole. Gamble was kind of known for those but I support her in her next endeavor anyway.