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:
Remote Sensing: As a standard action, a deity of rank 1 or higher can perceive everything within a radius of one mile per rank around any of its worshipers, holy sites, or other objects or locales sacred to the deity. This supernatural effect can also be centered on any place where someone speaks the deity’s name or title for up to 1 hour after the name is spoken, and at any location when an event related to the deity’s portfolio occurs (see the deity descriptions for details).
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.