There is only one way to make sure that you are not supporting a sweatshop. Even if something is made in the USA, that does not mean that it isn't made in a sweatshop. The U.S. Dept. of Labour estimated in 1996 that 50% of all U.S. apparel factories are sweartshops.
So how can I make sure I'm not supporting a sweatshop?
Easy. A goodwill store would be an excellent place to start. Oftentimes the clothes there are next to brand new and you can sometimes find vintage or expensive clothing there for like $2. And the money you spend there goes to the poor so no matter what, you're good!