Click the deceiver, and you can find "Train marine" button.
Common survivors don't have this one. You can divide survivors and deceivers easily in this way.
If you ambushed by deceiver, just run away as fast as you can.
Deceivers became human form when it did not see any players or survivors.
Just run fast and check if deceiver becomes human or not and kill it (if it becomes in human form)
Sorry for bad english. I'm not good at this