How to know if an object is autoreleased or not?
Basically, if you init, copy, or retain an object you are responsible for releasing it. If you don't, you are not responsible for releasing it.
Many classes provide methods of the form +className... that you can use to obtain a new instance of the class. Often referred to as “convenience constructors”, these methods create a new instance of the class, initialize it, and return it for you to use. Although you might think you are responsible for releasing objects created in this manner, that is not the case according to the ownership policy set out earlier. Because the class creates the new object, it is responsible for disposing of the new object.
The method signature itself tells you. The pattern to methods with signatures like "classNameWithData:data1:data2" is to return an alloc/init/autorelease instance of that thing. They are conveniences there so that you don't have to do it.
If you do not want an autorelease version of something, then do not instantiate them that way, and use the proper alloc/init, and release when you are done. This method is much more explicit, and a bit more error prone because if an exception is thrown your release block could get missed, but that is the price you pay for having it that way.