Be the wiser character. If you think that all humans are an inferior threat to nature but you need their help, your character shouldn't be surprised when they verify your assumptions. You "knew" they were problems going in and you still chose to work with them, so seeing evidence of it shouldn't change your approach.
Someone calls you an idiot? You're almost 60, you've seen people like them get killed by their own foolishness, and you're going to become a god. Say something dismissive and move on. Young, rash adventurers get offended by casual insults. Old, wise sages know not to listen.
Unless everyone at the table wants to play a PvP campaign, just play the grumpy, dismissive old dude who puts up with the others because they're necessary to his plans. It's fun for the other players to bicker with a curmudgeon; it's probably not fun for them to get into a drawn-out PvP combat with someone you admit is too powerful.
Firstly, it sounds as though your players are doing a lot of the heavy lifting for you in that they are being very easily manipulated without much effort on your part at all. But let's talk about manipulation.
Pa-pa-pa-poker face
Manipulation relies on getting people to do something without them knowing exactly why you want them to do it, or even that you want them to do it at all sometimes. Why? Because if they knew your reasons, they most likely wouldn't do it, and that doesn't help you. So lying. Your duke should agree to pay these adventurers anything they want if they do the tasks required of them. And he will lie. Oh will he lie. He will lie gleefully and to their faces, especially if they are silly enough to just blithely believe everything he says to them.
But what if they don't believe him...
Believable lying for beginners!
The best lies rely on two key concepts.
- A kernel of truth
- An appeal to emotion.
Let's go through them.
1) The more outlandish the lie, the less likely that someone will believe it. This seems like a common-sense thing, but people have a hard time with it. The best way to lie, then, is to distort or stretch the truth. Maybe the kobolds really do fear the dragon and want it dead because it could kill them all. It just also happens that a dead dragon means its horde is ripe for plundering, but the kobolds won't mention that they plan on taking all of that treasure for themselves. Why bring it up?
And an important thing to remember here: if you're playing someone who is actually manipulative, they will not seem stereotypically manipulative or as though they are lying. No long pauses or sideways glances, no hemming and hawing over details, no emphasizing exact wording ("I won't take your treasure." etc.) Lie. Lie right to their faces. Straight faced and unabashedly. It's amazing what people will buy if you just don't call attention to yourself and your weird behaviors.
2) Appeal to emotions. The players must care about some things. A manipulative person can tap into those desires. If your party is obviously in it for the wealth, maybe the duke talks about all the hidden treasure that would be "too much of a hassle" for the kobolds to dig out themselves, but the heroes are welcome to any of it they find along the way. If the heroes are sympathetic to the plight of the underprivileged, perhaps the thing the duke wants them to do is causing famine or disease, or is hurting the poorest of the community. "Really, it would be better for everyone if someone stopped it, but darn it I just don't have the resources to." Say anything to get them to agree. That's the point.
You lying bastard!
When the PCs do find out they've been had, there are one of two ways the duke can play it. He can try the "it was a misunderstanding" approach if he hopes to manipulate them again, but more than likely he will just be dismissive. He presumably has a contingency of kobold soldiers at his disposal, an army should be more than enough to take care of any PCs. Into the arena pits they go! (The duke may want to ask for more soldiers from his emperor while the PCs are away. He knows he lied to them and knows that they'll be pissed and knows they were capable of killing a dragon.) Don't make it a big show either. If the PCs call him out for his lying ways, he just accepts it. No reason to care what they think, they aren't kobolds and so they are inferior.
Best Answer
With their supernatural powers of remote sensing.
As it happens, there is a 3e sourcebook called Deities and Demigods which contains a wealth of material about deities, both from a creative perspective (how to design your own pantheons etc.) and a mechanical one - presenting rules for how to mechanically represent gods in 3e D&D. A 3.5e version was not published, but I figure this is close enough for your purposes...
In the rules laid out in this book, every deity has a Divine Rank, a numerical quantifier of their divine power. This mostly falls in the range 0-20, with 0 being a demigod, and 20 being an extremely powerful deity (it can go higher, but rank 21+ entities are described as being "beyond the ken of mortals" and don't take worshippers/grant powers as other gods do). The power of Remote Sensing is available to every deity of Divine Rank 1 or higher, as described on page 28:
This ability works across planes and through any magical shielding short of a block put up by another god. The deities aren't considered to have an infinite supply of attention - they can only concentrate on and actually perceive a limited number of places at once - but they can basically scry at will on anyone who worships them, mentions them, or happens to be near their temple, automatically perceiving everything that happens in a range of miles around the centre of their attention. This is how deities, as described by this sourcebook, keep up to date with what's going on in the world.
So what does this mean for your Paladin of Pelor?
Pelor, specifically, is given in this book as a "greater deity" with a Divine Rank of 17 (he's pretty powerful, as gods go). This grants him the ability to remotely sense 20 different places at once, and perceive everything that is happening within 17 miles of those places, which is a pretty astonishingly large area when you think about it. He doesn't need to sleep, and he can just cycle those 20 focuses of attention around the myriad of things he can sense. Checking in on his most favoured mortal servants - clerics and paladins - seems like an eminently worthwhile use of his sensing abilities.
While your transgressing Paladin cannot be certain that Pelor will be watching at any given time he's harassing orphans or whatever it is he's doing, it is overwhelmingly likely that if he keeps it up, Pelor will at some point be checking on him or someone/something within 17 miles of him and therefore catch him in the act - and presumably be most displeased by the scene unfolding.