My fortress seems to be glitching the pathing systems a bit and I end up with a bunch of dwarfs, mostly foreigners, who like to jump into a river and fall off a waterfall. That has left me with a bunch of line items on the dead list. Im a little paranoid that one of my dwarfs is on there and I missed it. How can I tell if something on that list is mine?
One way is to give all of your dwaves nick-names while they are still alive, then you can tell if a corpse is yours based on whether it has a nickname or not.
Obviously, that doesn't help when you already have mystery corpses strewn around.
If one of your dwarves dies where another of your dwarves can see them, you get a message report that they died. If no one sees them die, then a week later, you get a message that so-and-so has been missing for a week. (Or it might be after two weeks, can't remember). "Missing" means "dead, but with no witnesses". You can update the announcements.txt file to have it stop the game every time there's a 'died' or 'missing' report (and stop it from stopping the game and recentering the screen for every darn birth), but that's a separate question :). There's information about how to do that on the wiki.
Make blank slabs if you don't have any already. If/when you do, go to a craft-dwarf workshop, and choose engrave (I think it's alt-S, don't remember for sure.) It will give you a list of people that you can create tombstones for; any of your own dwarves that have died but have not been memorialized will show up at the top of the list. If you see a death on there that you know is NOT one of yours, then no name below that name is yours either.
Depending on how many dwarves you have (Or if you use the Dwarf Therapist utility), you can check the thoughts of your remaining dwarves to see if they are unhappy about losing someone or worried about someone missing. You can also check their relationships, to see if any are marked as "deceased". If one of your dwarves has a "deceased" relation that has the same name as a mystery corpse, it's probably one of yours.
if a ghost shows up, that means one of your own dwaves has died and not been properly memorialized. If you can't recover the body, then engraving a slab and building it will sufffice to put the spirit to rest. If no ghost shows up for an unburied/unmemorialized corpse within a year or so, it wasn't one of yours.