What skills rolls would you not require an action to use?

[SirVladamir]

I am curious what you DMs rule for various skills to be used without requiring an action/reaction.

In Xanathar's it says arcana check is a reaction to identify a spell when it is cast. But what about an existing magical effect, do you require the character use his action to identify it?

Other instances
1. Using any knowledge skill to determine information about a monster
2. Animal handling skill for various ride checks? controlling a mount not trained in combat when near combat, jumping over something
3. Investigation skill to spot an illusion/phantasm
4. acrobatics skill to avoid some fall damage or avoid an environmental effect
5. deception skill to act like you were affected by hold person (and how did you know it was a hold person unless you already used your reaction for an arcana check?)
6. insight skill to determine something. If a player has no reason to suspect something, his character would have no reason to use an action, but the character should have a passive chance, do you burn his reaction?

I'm sure there are others but these should get the discussion started.

[Chronos]

SpotPerception checks are often no-action, but investigation always requires an action.

[DeTess]

I'd say any skill that doesn't require physical intervention I wouldn't require the use of an action for. So things like healing, using tool proficiencies or investigation would require an action, and things like knowledge checks, perception or insight won't. Athletics or acrobatics are a bit in a grey area, so I'd change that depending on context. Acrobatics to avoid taking damage from a fall wouldn't take an action, for example, but bashing down a door obviously would.

[Pelle]

Depends on what they want to do with the skill, and how much of their turn they dedicate to it affects how I set the DC or if I give advantage.

Trying calm down a wild dog while you are fighting someone - high DC. If you stop fighting and focus on calming the dog, spendng your action on that - lower DC. Different tasks, different DC, judgement made in the situation.

[Man_Over_Game]

Reaction: Things happen around the player, and the player wants to react to it.
Action: The player wants to actively cause something to happen and is willing to invest into it.
No Action: The player wants to observe something without any investment or change.

Make sure that whatever you decide the event requires is *strictly worse* than any of these scenarios:

Bonus Action: to spot a hiding creature using Perception (Inquisitive Rogue)
Bonus Action: to investigate a clue using Investigation (Inquisitive Rogue)
Bonus Action: Use an Object (Thief Rogue).

Because of the fact that there are class abilities that provide a *benefit* to perform these actions with a resource, all other instances of performing those actions must use more expensive resources (so spotting a hiding creature should be an Action).

[furby076]

Make sure that whatever you decide the event requires is *strictly worse* than any of these scenarios:

Bonus Action: to spot a hiding creature using Perception (Inquisitive Rogue)
Bonus Action: to investigate a clue using Investigation (Inquisitive Rogue)
Bonus Action: Use an Object (Thief Rogue).

Because of the fact that there are class abilities that provide a *benefit* to perform these actions with a resource, all other instances of performing those actions must use more expensive resources (so spotting a hiding creature should be an Action).

I don't know about the worse thing. Typically someone with a special ability either gets proficiency to bonus (or double in some rogue cases) or the ability spells out extra effects. Adding my own, while may make sense to me, is house rules. It would require keeping a track of those and might be a bit much.

[Slipperychicken]

If it doesn't take physical action or time beyond a quick glance at something, then it shouldn't take any actions. At most I'd count it as consuming a free object-interaction.

Rapid split-second movement to avoid harm (i.e. acrobatics to minimize fall damage) should be a reaction.

Controlling a mount is part of movement for a proficient rider. In combat with a non-war-trained mount, I'd require a DC 10 ride to fight with it (i.e. rider attacks while mount moves) or guide with the knees (i.e. ride without use of the hands); otherwise the rider must spend his action calming the mount or he is thrown and/or the mount spooks and bolts. With a war-trained mount, no extra action of any type needed.

Reaction to identify a magical effect is BS. I'd rule it as a non-action; if your roll is successful you know instantly and may counterspell, drop prone, or otherwise react as appropriate. If someone fails the roll and wants to wrack their brain about it the next day they can try again, or succeed automatically if they have a reference of some form.