How can I determine if an NPC is available for marriage?

In the vein of Are inter-racial marriages frowned upon? - Pete Hines, Bethesda's VP of PR and Marketing, when asked if Skyrim would support gay marriage said the following:

You can marry anyone...Not any NPC, just mean any male or female.

So obviously some NPCs are off the list. I assume I can't marry Jarl Balgruuf, but what about Irileth? What about the Graybeards? What about already-married NPCs? Is it any "important" NPC, or is it more or less than that?

Has the Skyrim Creation Kit shed any light on how to determine this?

NPCs who can be married will be part of a faction, marriage-faction.

The console command addfac 19809 1 while having an NPC selected will make them eligible for marriage.

For scanning through info in the Construction Kit, look out for

marriage-faction

and the faction ID

19809

According to The Elder Scrolls Wiki - Marriage you first need a certain amulet to see the relevant marriage dialogue options.

That site also has a list of eligible marriage partners, so I'm not sure it's true that you can marry just anyone you encounter.

There are a few people you can marry some in small villages or mills others in the city. You can't marry just anyone, and you can only marry once. The first person that will marry you is the Riverwood trader girl, as you do the quest that is related to her, but there are others. Largely it doesn't matter who you marry, the benefits are all the same. She will cook a meal a day for you and give you the proceeds from the shop she sets up (100gp) you can also trade with her. If you marry one of the available NPC's that will adventure with you then they can still adventure with you when they are your wife.

All you need is the Amulet of Mara, and a little instruction from the priest and you are good to go and find some woman, man, Argonian, Dark Elf, High Elf that is willing to be your husband/wife or what have you.