My question is, how can we understand/tell the difference between a dream from Allah and a dream from Shaytaan?
1- From Allah
2- From Shaytaan
3- From oneself
Now the dreams from Allah can be understood as those good dreams which give you good feelings/tidings that something good is about or is going to happen. The dreams from Shaytaan can be understood as those dreams which do nothing but frighten you, in other words nightmare. Now simply put the difference between them, is the difference between good and bad dreams.
Note that a person may have a dream for example of Yaumul Qiyama which is extremely frightful but every event in that dream can be supported by authentic evidences, then that dream is not from Shaytaan but from Allah, and can be understood as Wake up Call. And Allah knows best.