The readied action should trigger if the creature believes a spell has been cast
The description for the Ready Action says:
First, you decide what perceivable circumstance will trigger your reaction.
So, the trigger must be perceivable. Spells do not have effects before they are cast so the only things you can observe are the somatic, verbal, and handling of the material components. Then, of course you might be able to observe the actual effects of a cast spell.
Note: the readied action will always take place after the trigger has completed
If the trigger such as "when the enemy casts a spell" is used, then the readied action takes place after the spellcasting (or fake spellcasting) has been completed. The way the rules are written, no wording of a readied action can interrupt the trigger. It seems to be a matter of debate whether any readied action can be made such that the trigger would happen before the spell is completed. See Timing of the Ready action when the trigger is associated with spell casting? and Is it possible to interrupt spellcasting? for more discussion on this topic.
Let us examine how the readied action will work with the trigger "when the enemy casts a spell" for both a real spell and a faked spell.
Real spell (casting and effects perceivable) - would trigger
This is the one of the clear-cut cases. If one can clearly observe the casting and the effects of a spell, clearly a creature would be able to see that a spell has been cast. Thus, after the spell has been cast completely, the readied action would be able to be taken.
Real spell (casting and effects imperceivable) - would not trigger
This is the other clear-cut case. It is possible to make the casting of a spell imperceptible through the use of metamagic and/or other spells (eg silence). Pairing that with a spell whose effects were not observable, results in a spell that is completely imperceivable. Thus, there is nothing that would trigger a readied action.
Real spell (only effects perceivable) - would trigger
If the spell was made to have imperceivable casting but perceivable effects the readied action would trigger because the spell effects would obviously be a sign that a spell had been cast.
Real spell (only casting perceivable) - might trigger
At this point, things become murkier. If the caster makes the casting of the spell obvious but there are no observable effects of the spell the readied action may trigger.
There is no real rules guidance for a case like this that I can find. However, it seems that a reasonable way to handle this would be for the DM to determine if the character could reasonably believe that a spell was cast. This could be through an arcana check or something similar. If the DM decides that they do suspect a spell has been cast, the readied action could trigger even if the effects if the spell are not perceivable.
This makes sense from a narrative sense because a character is relying on their own knowledge and senses to watch for the trigger. If they suspect from their observation that the trigger has occurred, they will then proceed to carry out their planned action.
Fake "spell" (only "casting" perceivable) - might trigger
In this case, the spellcaster would fake words and gestures and the handling of material components, but nothing would happen. Since they are not casting a spell, there are no spell effects. Since there are no spell effects, this case is murky just like the last one.
In fact, from the action-readier's point of view, this case is identical to the last case (only casting perceivable) since without an observable spell effect there is no way to be certain that a spell was cast.
Thus, similarly the DM should determine whether the character believes the trigger has occurred. In this case, this might involve an insight check or arcana check opposed against a deception from the caster. The DM can use whatever method of determination that seems best for the scenario. The important thing is that, if the character would truly believe the trigger to have occurred, they should be able to take their readied action. If the character recognizes that the caster is faking, then the character knows that the trigger condition has not occurred and will not take their readied action.
Other trigger wordings probably will work better
If one wanted to craft a trigger that worked when a spellcaster is casting a spell and when they are pretending to cast one, then you must use a different wording than "when the enemy casts a spell".
A more workable trigger if one wanted to achieve this goal might be "When someone looks like they are casting a spell" or "when the wizard raises his wand" or "when the spellcaster makes a movement that appears aggressive to me".
Since there are so many circumstances that can occur, it is going to be exceedingly difficult to craft a trigger that will work with certainty, but the above will certainly work in some circumstances and definitely will work if the spellcaster is pretending to cast.
Thus, through careful wording, you can trigger a readied action on either real and/or faked spellcasting.