We know that there is a spell used to reveal things we can't necessarily see at that moment--Specialis Revelio, Hominum Revelio, and by extension, Hermione's Revealer (opposite of an eraser) from CoS. What do these spells actually do? We never see one work. Harry has not mastered Specialis as of HBP, when they are supposed to use it in Potions. Hermione's Hominum revealed that there were no people around in DH, so we didn't see it then. The DE's Hominum in the Lovegoods' house in DH had a physical effect on Harry and Hermione.
So what does the caster of the spell actually experience, and how do they receive the information?
My opinion about revelaspells
The books gave me the idea that a revelaspell, specifically the ones that tell you if a person or thing is present in a house, takes a kind of imprint of anyone it is "revealing" and sends the shape back to the person who cast the spell, in the form of a smoky figure or shadow.
Take for example Dumbledore's use of an unidentified object to reveal the nature of the snake that attacked Mr. Weasley in OotP:
"The instrument tinkled into life at once with rhythmic clinking noises. Tiny puffs of pale green smoke issued from the miniscule silver tube at the top. Dumbledore watched the smoke closely, his brow furrowed, and after a few seconds, the tiny puffs became a steady stream of smoke that thickened and coiled in the air....A serpent's head grew out of the end of it, opening its mouth wide." (p. 470 of hardcover edition)
Here it is a smoke figure that reveals the essence of Harry's dream, although this case isn't exactly like one where a wizard is trying to reveal the presence of someone.
Then there's the passage in DH when the actual homenum revelio is used on Harry, Ron, and Hermione while they're at the Lovegood's house:
"Harry heard Hermione gasp, and he had the odd sensation that something was swooping low over him, immersing his body in its shadow." (p. 421 of hardcover edition)
This gives me the impression that casting a revelaspell sends an invisible force up to take an imprint or mold of any humans in a house or building, and then sends back a sort of negative to the caster.
But this is all just my interpretation of revealings that occurred in the books. Other opinions would be equally supportable.
I kind of figured that Specialis Revelio and Hermione's Revealer were rarely functional, kind of like the Esc key.
For Hominum Revelio, I would think the wand would light up or make sparks if someone was around. The wand would definitely signal in a silent way, because otherwise it would just give away the caster's location.
um, this might sound a bit funny but when I was reading about Hominum Revelio in DH, I had assumed that the caster might feel vibrations or something if someone was present. If that sounds too far-fetched and non-magical, You can have the same effect on the caster that the trio had when the spell was used around them. If not even this, then you could always say that the caster just feels it, as in just gets to know... I don't know; I know I'm sounding very confused, but I hope you understood what I meant.
I just thought it was sort of similar to a bat's echolocation. Like witch6 said, it would have vibrations coming back to the caster. For Hominum Revelio maybe the amount of vibrations would tell you how many people were in the vicinity. I suppose it also depends on how specific the information needs to be. We don't know if the spell tells you specifically who is there, or if it just tells you that someone is there.
Hope that makes sense!
Originally Posted by Hermione, DH, chapter 9
The above, I believe, should constitute sufficient evidence that Homenum Revelio only reveals the presence of humans in the area, but does not provide any more specific information.
Originally Posted by Travers, DH, chapter 21
As for the effect, I subscribe by Ayesha's idea of 'echolocation'.