CADSOFT EAGLE libraries searching best practices
What is the best practice to find libraries for Eagle?



First you must be sure that your device is not in eagle libraries using good search, For example if you want to add 7805 5v regulator, You must search like this:
*7805*
That because certain manufacturer have their own extension that you don't know.
If you didn't find your device or reasonable alternative (you can use for example atmega8 device instead of atmega328 device.Try to use famous public libraries produced by: 


SparkFun Eagle Lib - AdaFruit Eagle Lib - Dangerous Prototype Eagle Lib - Element14 Eagle Cad Lib Search 


Also a good practice is to memorize if the part you want to use is used in previous open source board, If answer is Yes then go and export used parts there from ULP.
When you build your own library keep in mind that there is a reference library in eagle called Ref-packages.lib that have most of electronics packages which make create new libraries much easier.



  How can I be sure that the library I found is a good quality?


Just print footprint on a paper and make sure that every thing is OK, or you can make sure from eagle that physical footprint is typical to part description from datasheet.