Easy thing is just to have skills like gather info but if you actually want something specific that other heralds might not have done how about:
undetectable alignment and disguise self. You aproach them first as the herald and get them for the job if they seem honest and worthy enough. Then later that night you show up disguised and undetectable alignmented as someone else and tell them you're send by another power in the kingdom who wants the mission to fail. You offer them large amounts of moneys if they agree to sabotage the mission. Judging on their reaction and what they say to your (regular form) herald the next day you have more info on their honour and such.
undetectable alignment and disguise self. You aproach them first as the herald and get them for the job if they seem honest and worthy enough. Then later that night you show up disguised and undetectable alignmented as someone else and tell them you're send by another power in the kingdom who wants the mission to fail. You offer them large amounts of moneys if they agree to sabotage the mission. Judging on their reaction and what they say to your (regular form) herald the next day you have more info on their honour and such.