The short answer is that you can't, any more than you could automatically know the owner of something you pick up IRL. If I might take a guess, I'd suppose you either have something you picked up that was abandoned in an NPC shop or something you bought from a production faction.
In the first case, you wouldn't expect to know the owner of, say, an abandoned backpack you found in real life, would you? Nor can you know it in the game (unless someone's name is "written" on it -- in the item name).
In the second case, if you posess something from a production faction you might be able to form a good guess as to who owns it by finding out which faction(s) produce that item. You might even get lucky and find that only one faction produces it and therefore have a pretty likely answer. Unfortunately a lot of factions aren't making items over yet because of the size of inventory they'd have to poke through to find individual items. Best thing you could do is name the item with your handle, drop the faction a polite note, and hope for the best.
Also, iirc Kyle Rainer has said he's still got another script he's getting ready to run to address item ownership once he makes sure it won't crash the server. Yayz for teh Bunny.
Good point on not knowing who 'owns' an item in real life. But I guess that brings up another question regarding an inventory list. Why does a list of items exist in my inventory? It seems that the only way to know that you own or have something is to physically have it in your possession, or in a secured location.
In my inventory list, there are a few items that are spread around the galaxy, and some of them are being carried by other people. Do I still own those? In real life I would no longer own those or have anyway of retrieving them. Yet my inventory list shows who has it and where it is.
I only bring this up because when I first started, I picked up an item that belonged to someone else (didn't know any better at the time). At some point, he changed the name of the item to 'STOLEN FROM XXXX BY AMP THANE THIEF' (I can't remember the guy's name). I contacted him and paid him for the item, since I didn't want to be a thief. But, this would never happen in real life.
Light speed is too slow! We're going to have to go right to...Ludicrous speed.
There are plans to "fix" inventory, to prevent it from being possible to see who's carrying around a randomly-picked-up item on the other side of the galaxy. Until then, assume that anyone doing that is a noob, and people are generally going to ignore them.
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."