Well, you'd really have to get the relationship value to almost impossibly high levels for him to use it consistently. As for how you can tell... you can't, just keep improving it by paying him more gil, never instantly dismissing him, never letting him die, reseting whenever he uses Diagoro etc.
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Signature credited to Nathan (or whatever he decides to call himself next)MSN address: yamiken (at) hotmail (dot) co (dot) uk
[size=6](You know the drill, remove the spaces, replace the at with @ and the dot