If you are actually casting a spell, your scheme won't work. If instead you are attempting to fool an enemy caster into thinking you are casting a spell, it may or may not work, but it's up to the DM.
By default the rules do not require the counterspelling spellcaster to know the spell being cast, or it's level in order to counterspell, so your scheme won't work in general.
Counterspell states:
You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, it's spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect.
As a result, you can't trick them into counterspelling the "wrong" spell, if they cast Counterspell on you and you are casting a spell...then your spell will be countered if it is 3rd level or lower, or faces a chance of being countered if it is 4th level or higher.
That being said, the enemy caster won't know what spell you are casting, or even that you are/are not casting a spell in general. By a corollary, they won't know that you are "pretending" to cast a spell but not actually doing so (if you can pretend well enough)!
[...] Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. [...]
(Player's Handbook: Casting a Spell > Targets)
So you might be able to fool them into wasting a counterspell on a "spell" once (Fool me Once and all that).
What does it take for a spell to be perceived?
In order for your ruse to work your spell/"spell" needs to be perceptible to be counterspelled. Absence some special sense (like that afforded by Detect Magic possibly), you aren't even guaranteed to know that a spell is being cast by the caster.
Xanathar's Guide to Everything, provides additional optional rules to cover the circumstance of perceiving a caster at work (expanding upon the PHB quote above):
[...]
To be perceptible, the casting of a spell must involve a verbal, somatic or material component.
[...]
If the need for a spell's components has been removed by a special ability, such as the sorcerer's Subtle Spell feature [...] the casting of the spell is imperceptible.
It also has optional rules on identifying a spell being cast:
[...]
To do so, a character can use their reaction to identify a spell as it's being cast, or they can use an an action on their turn to identify a spell by it's effect after it is cast.
If the character perceived the casting, the spell's effect, or both, the character can make an Intelligence (Arcana) check with the reaction or action. The DC equals 15 + the spell's level. If the spell is cast as a class spell and the character is a member of that class, the check is made with advantage.
[...]
Being able to cast spells doesn't by itself make you adept at deducing exactly what others are doing when they cast their spells.
So what happens if I pretend to cast a spell using my action/bonus action, but I'm not actually casting one?
It's situational, and is up to the DM.
Your character can do things not covered by the actions in this chapter [...].
[...]
When you describe an action not detailed elsewhere in the rules, the DM tells you whether that action is possible and what kind of roll you need to make, if any, to determine success or failure.
(Player's Handbook: Actions in Combat > Improvising an Action)
Assuming the DM allows you to attempt this then you would almost certainly need to make either a Deception or a Performance check using your action/bonus action as appropriate to successfully deceive the caster (likely contested by their passive Insight or Perception, since they aren't "actively" trying to deduce that you are casting a spell).
Your Charisma (Deception) check determines whether you can convincingly hide the truth, either verbally or through your actions. [...]
(Player's Handbook: Using Ability Scores > Using Each Ability)
Your Charisma (Performance) check determines how well you can delight an audience with music, dance, acting1, storytelling, or some other form of entertainment.
(Player's Handbook: Using Ability Scores > Using Each Ability)
Assuming you pull that contest off, then the enemy caster has three options:
- Do nothing and let the "spell" go off
- Attempt to counterspell the "spell" and waste the counterspell slot (the creature casting the spell is the target of counterspell, not the "spell" itself!)
- Attempt to identify the "spell" using the optional rules from XGtE so another caster can counterspell the "spell" appropriately. In this case, since you aren't actually casting a spell, the DC for identification would be a flat 15. You DM may alternatively allow the DC to be your Deception/Performance check result.
Which the caster chooses is up to your DM. The outcomes from their perspective could be the following (linked to the relevant action they took to your casting):
- They think you cast a spell that either didn't work (or that didn't produce a perceptible effect)
- They think they successfully counterspelled your "spell" (or possibly realise they wasted the slot...which might make them less happy to counterspell a "real" spell cast by you)
- Waste a reaction and
- Beat the DC and realise you weren't casting a spell
- Be unable to deduce the "spell" you are casting
1: Acting as though you are casting a spell is still acting!
