I took a long break from Skyrim and have recently picked it back up. Played quite a bit from release until late 2012 when I started getting into other games. Across about 400 hours of play, I ended up with three main characters (in descending order of level/hours spent: sneakthief/assassin, DW no-magic warrior, pure mage. The mage is a relative baby, but the other two I played quite a bit (and mostly in that first year, so it's been a while). The point is, I've forgotten now what all I did on those first two characters. For example, I know I've at least started the Thieves' Guild and Dark Brotherhood on the sneakthief, and the Companions on the warrior (he has wolf form, that much I can tell). My question -- is there a way I can, relatively quickly, tell whether I've completed those questlines (which I would assume make me the leader of the respective guild) or how far along I am? A handy ESC menu screen or something would be great, but something in-game (e.g., someone to talk to whose reaction would make it clear) is ofc doable as well. Thanks!

bring up the "journal" and look at your completed quests.

This. Just search your journal for each of your characters.

Completed quests! I always forget that the Journal tracks those as well as active/uncompleted. Muchas grassy ass.

In each guild I think you go from one quest to the other immediately, so if you have a list of names for each guild questline then you should be able to find which you've done