Can a medium tell if he/she is talking to a demon or bad entity?

Sometimes the energy will feel differently to different people, depending on how they interpret it.

For example, if someone says a statement, that statement can make you sick to your stomach (figuratively speaking). Even though other people won't be as bothered by the same statement.

What YOU interpret as evil is not necessarily what others will interpret as evil.

Sometimes people who are at war with each other interpret each other as "evil", but we all know that they are not, they are just people, just like us, both good and bad.

One person could do something bad and sometimes some good things. So is he a bad person /spirit or not?

Sometimes something that is very DIFFERENT from you, from your energy may feel unpleasant... but it doesn't mean it's a bad spirit.

But basically most people would probably just go with their feelings if they feel good, they perceive the Spirit to be good. If they get a bad vibe, they perceive the spirit as "bad" (whatever that means to them).

Mystica has great insight. When it comes to a demon, test, and test again. If an angel is talking to you. You can ask for a sign. Better to go mute for 6 months than insane. Unless you know that you are ordained by a holy man/woman, just protect yourself and get out. Seek help if it follows you. 2-3 in unison people who bless you in faith will encourage it to move on. They don't have to be good at their religious practice. But the key is faith.

[Sammykitty]

I'm an empath so I'm very sensitive to emotions which makes it a little easier to pick out a negative entity based on their deep, dark, energy they emit. When a spirit is malicious I usually feel a pressure in my chest like it's trying to take my breath. At first, when my abilities were just manifesting, I would get this feeling about any spirit but now it's only when the spirits, or entity, is negative. I always carry black tourmaline in my pocket when I know I'm going somewhere with high traffic just in case I run into something negative.

I suspect bad is what you make of it. I have had a spirit peel off me who was very unhappy, to the point of being demonic in her influence. I say her, because that was the nature I saw. But she also was lonely and desperate, as she came back a couple weeks later, having no where to go. I did not have enough presence to help her. As DEMONS go, meaning forces that can change you as basic, I doubt they exist. You run the show, always, and agree to forget your power and who you are only with your willingness.
So it is a good idea to know who you are.