Certain spells and conditions refer to "Magical Effects". In our group the use of the Circle of Power spell is my main concern but the Charmed condition or Antimagic Zones also include those Effects. So what are they? There are examples made but they are far from exhaustive.

One special concern is that during my first go at dming I plan the group to encounter some Ghosts. Now I wonder whether horrifying visage and/or possession are effects circle of Power protects against. I tend to say no but I am not sure. My reasoning is that a ghost would probably be able to possess and Show his horrific face in an antimagic Zone. Although I would rule that he could not use those Powers against anybody who had charmed him (If He could be charmed), circle of Power is closer to an Antimagic Zone in my opinion.

A feature, trait or something else is magical when the description says so. For instance, the Possession action of a ghost is not magical and can be dispelled with dispel evil and good or similar spells.

Sage Advice Compendium explains how to determine whether something is magical:

Is the breath weapon of a dragon magical? If you cast antimagic field, don armor of invulnerability, or use another feature of the game that protects against magical or nonmagical effects, you might ask yourself, “Will this protect me against a dragon’s breath?” The breath weapon of a typical dragon isn’t considered magical, so antimagic field won’t help you but armor of invulnerability will.

You might be thinking, “Dragons seem pretty magical to me.” And yes, they are extraordinary! Their description even says they’re magical. But our game makes a distinction between two types of magic:

• the background magic that is part of the D&D multiverse’s physics and the physiology of many D&D creatures
• the concentrated magical energy that is contained in a magic item or channeled to create a spell or other focused magical effect

In D&D, the first type of magic is part of nature. It is no more dispellable than the wind. A monster like a dragon exists because of that magic-enhanced nature. The second type of magic is what the rules are concerned about. When a rule refers to something being magical, it’s referring to that second type. Determining whether a game feature is magical is straightforward. Ask yourself these questions about the feature:

• Is it a magic item?
• Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
• Is it a spell attack?
• Is it fueled by the use of spell slots?
• Does its description say it’s magical?

If your answer to any of those questions is yes, the feature is magical.

Let’s look at a white dragon’s Cold Breath and ask ourselves those questions. First, Cold Breath isn’t a magic item. Second, its description mentions no spell. Third, it’s not a spell attack. Fourth, the word “magical” appears nowhere in its description. Our conclusion: Cold Breath is not considered a magical game effect, even though we know that dragons are amazing, supernatural beings.

Thanks. I did not find that and it sounds like a good guideline.

As a sidenote, the Nystul's Magic Aura spell refers to the Paladin's Divine Sense as an example for a Magical effect and you would answer all of the above questions with no. No rule without exception it seems.

I'd consider that an error in Nystul's Magic Aura. Racial traits, spells, class features, feats, and other exceptional game elements are meant to stand on their own; Nystul's Magic Aura shouldn't affect your ruling on how Divine Sense works. Otherwise, you'd have to check the whole book any time you make a ruling on a specific feature.

It's pretty clear Divine Sense is extraordinary and it's intended to be fooled by Nystul's Magic Aura but I wouldn't consider it magical.