Since reinstalling Windows 7 -- and Sims 2 -- I've become a bit paranoid about my downloads. Or more specifically, what downloads I cannot delete without corrupting the game.

HOW do I find out if a CC object that I drop into my downloads folder is an NPC and should not be removed? (preferably BEFORE I've put it in my downloads folder)
Is there a list of known CC that uses NPCs? (I've tried searching but all I can find are Wiki pages talking about Maxis NPCs and random posts mentioning rideable bicycles, remote controlled cars, a shark & dolphin, some farm animals.)
If not, can someone in the know start a list on Wiki that players can add to as more NPC content becomes known?

For now, do I just assume that anything that moves or is animal-related contains an NPC?
Water wheel that creates milled bags of flour
Cows that produce milk
Chickens that produce eggs
All farm animals
Flying birds
Vehicles Sims drive
Rideable wheelchairs
Downloaded Dogs & Cats

No static animated things probably don't contain NPCs. You need the NPC for the thing to move around - that's why they are used. You don't need to worry about the remote controlled car (unless it's a CC one) because that's in the game already.

From your list and what I know of:
Cows - only if they move. If the milk producing ones don't move and are from Plumbob Keep, I think they are ok
Chickens - again I think the PBK ones are ok
Farm animals - PBK OK
With the animals, it's generally, AFAIK, the ones from Affinity Sims that have NPCs - made by Rebecca

Birds - yes
Also horses - rideable version as well as other
Other animals include elephants and other 'zoo' type animals

Don't know about the wheelchairs

Cats and dogs and vehicles should be just fine as long as they're made as variants of the EAxis versions (can't imagine why you'd make them any other way).

Bumper Cars
Surfing mod
These both use NPCs (though are great mods)

There are others. It's not a type of mod I use much though there are some nice things that use that technique.

Chris Hatch, a while back, made a complementary mod which is designed to be a central controller for this type of thing. His idea was to create a controller mod with only ONE NPC which worked for all of them. I read about it and made note and have the link somewhere. I only have the surfing mod in my game atm and so haven't got round to looking at that in detail. Might be worth a look though if you want to use that sort of thing.

I understand the paranoia!!!

Sun & Moon over at Plumbob Keep make animated stuff (including animals) that are not NPC-based. Just so you have an alternative.

Thanks, SciBirg!
I have a feeling that I'm going to be spending some time on PlumbBob Keep. That name keeps coming up as I search for stuff to start my historical -- though not Medieval game.