One of the most important factors for determining whether a spirit is good or evil is the level of development of the signal. Their language is also determining factor. It always corresponds to their level of development.
We can judge the nature of a spirit in the same way we judge people. So for example, a spirit might not be at a high intellectual level but may possess very high moral standards.
And vice versa - a spirit might not be very erudite but may be at a very low level morally.
There are several methods for determining whether the spirit we're sensing and/or seeing is benevolent or malevolent. Firstly is the feeling that we get in the presence of the ghost. A good feeling is a guarantee that it possesses numerous virtues.
Spirits' actions directly reveal the feelings which lead them to commit those actions. If they are good - the spirit is also good, if they are bad - so is the spirit.
By presumption, benevolent spirits can say and do only benign things. They cannot commit evil deeds, while malevolent spirits can hide a subtle plot behind a mask of good intentions.
Supreme spirits use only clear, noble-minded language. They don't bet on trivialities, they speak modestly, without flaunting superiority and knowledge.
The words of plain, lower spirits, contain obvious vulgarity, complacency and lies - a reflection of human passions. If you sense this type of behavior from such a spirit, while they introduce themselves under a respected name, know that you are most likely being tricked.
The form which the spirit or ghost presents themselves in is not a reflection of their overall inner power. Listen to their words, remember that any deviation from reason and logic would mean that the spirit is evil.
Benign spirits impart their wisdom. If you ask them something that they do not know, they will remain quiet and search for the answer along with you. In contrast, malicious spirits pretend to be omniscient and competent on all matters. Often, their wisdom is a lie.
If a given spirit tries to order you around - then it is evil. Good spirits advise but never impose themselves. They guide but never dominate over every little step that you take.