Often when we speak, we want to put more emphasis on a single grammatical element. The choice of the various "voices" is based on your interest in one of the following:

• the subject (use active voice)
• the direct object (use passive SE or passive voice)
• the verb itself (use impersonal or passive SE)

If you want to stress that the action referred to is completed, and the fact that the object is in the state of completion, then use the resultant state structure with ESTAR.

Impersonal Se often is equivalent to or overlaps the Passive Se. In other words, you can often interpret the same sentence in Spanish as passive or impersonal. This is not the case in English, so it causes confusion for English speakers. My advice is to try to let go of that anxiety--since there isn't a difference, it doesn't matter if you can't tell which one it is.

The few cases where the Impersonal Se is the only possible interpretation, the Active voice form of the statement has NO DIRECT OBJECT. The verb is either a "one ball" verb, or it is being used as a "one ball" verb.

• ¿Cómo se dice?
  How is it said OR How does one say it?

This one has an object, so it is both ways. Converted to active voice it is:

• ¿Cómo lo dice uno?
• No se habla en la biblioteca.
  One doesn't speak in the library.

There is no object in the active voice form of the sentence:

• Uno no habla en la biblioteca.

Hablar is being used as a "one ball" verb.

However, you can add an object:

• No se habla inglés en la clase.
  One doesn't speak English in the class. OR English isn't spoken in the class.
• Uno no habla inglés en la clase.

Notice that the difference between the two cases is that the first stresses the prohibition against ALL speaking. The verb is the ONLY focus. The second one has an emphasis on both the verb, but more importantly the English language.