"Ok, here's a question for you: I go to thrift stores a lot but tend to avoid the jeans section, I have trouble determining the age of a pair of Levi's for example. Can you share a few pointers on what to look for? The patch? The brand of the zipper? The rivets?"
First of all, do not avoid the jeans section.
Secondly there are some give aways. Here are some in non particular order.
1) Do the jeans have selvage? Turn the cuff - it's fast and effecient.
2) If they are Levi's jeans, does the red tab or orange tab on the back pocket spell LEVIS and not Levis. I'm refering to a thing known as "Big E", which was how LEVIS was spelled until 1971, when they changed the capital E to a small e.
3)Look for defunct jeans brands like Foremost (a JC Penney brand), Ranchcraft (Also a JC Penney brand), Key, Washington Dee Cee, Can't Bust 'em, Buckaroo by Big Smith, Roebucks - the list goes on and on.
4) If you are looking for zippers, the zippers you should be looking for are Talon (especially Talon 42), Scovill, Ideal, Gripper Zipper.
This post may be of some help
5) Knowing this timeline from Levi's is good when you need to date jeans. Just look at the differences in arcuates (the iconic stitching on the back pockets) for instance.
This post may be of some help. And if you need some help, I'll gladly give some in depth explanations to understanding it. But please remember that there can be variations depending on the factory used even back then.
Finally I would recommend you look on the inside tags. If they are "made in USA", they are vintage and probably a lot more interesting than most new Levi's jeans. But try them on, so you don't end up in a pair of boot cut jeans - unless you are rocking a full "Freaks&Geeks" attire....