State or process? Zustandspassiv
Sein plus Partizip II describes a resulting state. Werden plus Partizip II describes the process or action. Compare the already closed door with the door currently being closed.
Understand the idea
Process passive describes an action affecting something. State passive describes a resulting condition. The difference is visible in werden versus sein, but context is necessary because some participles also behave like ordinary adjectives.
Build it step by step
Use werden plus participle when focusing on the action or its progress. Use sein plus participle for the resulting state where that reading is appropriate. Time expressions and a surrounding explanation can make the contrast clear. For past time, the auxiliary changes while the action/state distinction remains.
See it in a sentence
Die Tür ist seit einer Stunde geschlossen.
Die Fenster sind bereits geöffnet.
A useful memory aid
Imagine a photograph and a short video: a photograph can show a resulting condition, while the video can show the action producing it. Use this as a guide, not as a test for every adjective.
A common trap
Do not choose wird just because the sentence contains a past participle. A finished result may call for ist; an ongoing process may call for wird.