Jillian,
I think that there is a thread on thrifting that many of us responded to but I do not know how to find threads.
A few answers;
1.I know the better ones which are usually in an urban area. Just go in and browse around, you can get an idea. I have a Savers right near me. It is right next to grocery store. I hate to grocery shop. My husband loves to. I shop at Savers at least once a week. Friday late in the day is a good time, Also mid -week. Weekends are not worth it . All the good stuff is picked over.
2. I have a list of things that I am always on the lookout for in my head. After I did a total closet organization, I noted the gaps and what needs replacement. I usually just go to those departments. I scan other areas and am really fast at noticing good color and quality. I also shop at many types of non thrift stores. I know what is out there and what I can easily thrift and what is worth spending full price on because I never see it at thrift stores. Accessories are the best finds. Scarfs, bags and fashion jewelry.
I usually avoid knits, they pill. Jackets and blazers are always a good thing to look for. I have gotten really picky even if the item is dirt cheap. I teach at an art school so I often like to buy things that I do not have to be really careful with. The rain boots are such an item, I will wear them in bad weather and even if they last just this season, that is ok. Trendy but not expensive and a dark color that can withstand charcoal dust.
I am looking for a classic really good pair of cognac boots and a great brown motojacket. I have not found either in any stores but will keep trying. Doubt that those will be thrifted but who knows?
I have been thrifting since high school and get lots of props and costumes for my painting. As an artist who loves fashion it has enabled me to buy clothing that I would not be able to afford otherwise. I also have bought and sold items to vintage stores. I have a good idea of what is really a great find . Items have to be in perfect or near perfect condition and current or retro. It is a hobby that I love. You have to go often and leave with nothing most of the time. You need patience.
Hope that helps.