[RPG] Can NPCs tell I am casting Command

There are a number of different strategies one can take (besides the possibly obvious: remove material spell components/focuses/holy symbols/etc.):

1. Make the jail special
   

• Suppress sound (e.g., permanent or ongoing silence) to eliminate verbal spell components

• Suppress light so that the spell caster cannot, for example, target individuals or locations via line-of-sight (e.g., use illusions over doors or windows to block line of sight).

• Suppress divination (e.g., permanent or ongoing nondetection) so that the spell caster’s buddies cannot scry in order to pop in via, say, teleport, and rescue them.

• The jail is built in an area that "naturally" suppresses, hampers or distorts magic.

• The jail is constructed using permanent or ongoing anti-magic spells

NOTE: Following up on one of SevenSidedDie’s comments on the original question, spell caster-proof jails may be rare enough and expensive enough things, that one would not expect small towns, or even necessarily any particular city to have the capacity to hold a spell caster (although this will depend somewhat on how common spell casters are in the campaign setting, for example, if every small town has a few high level casters, then these kinds of jail enhancements might be common). This kind of scarcity might make the possession of a spell caster-proof jail a special resource for a region, or even between nations/principalities/city states/etc. where extradition treaties, rendition treaties, or even trade treaties are negotiated around arranging the incarceration of a spell caster (imagine a side narrative about arranging the transfer of a incarcerated spell caster: fun!).

2. Alter the incarcerated spell caster
   

• Magically compel the spell caster not to use spells while incarcerated in jail (e.g., a geas, some really nasty custom curse, mark of justice, etc.).

• Make the spell caster wear some difficult to remove (probably homebrew) magic item that suppresses somatic movements, sounds, or even magic.

• Tag the incarcerated caster with difficult to remove tracking and summoning magic, and make them aware that there will be sanctions for jailbreaks.
  

• Place the spell caster in some kind of suspended animation (e.g., the vanish sink spell of 1st edition AD&D).

• Drug the spell caster so that they are incapable of casting (per orlp’s excellent comment).

3. Have the jail include guardians who are practiced at containing spell casters.
   

• Abjurationist wardens

• Mage-slayer wardens (perhaps equipped for and specializing in non-lethal combat and restraint)

• Wardens equipped with anti-magic devices

• Wardens that are magic-suppressing, magic resistant, or magic immune monsters or constructs.

4. Make compelling appeals to the incarcerated spell caster’s reason, morality or emotions

• Threaten the spell caster’s familiar, family, colleagues, or benefactor with sanctions (e.g., your family pays a huge fine if you jail break; your familiar will be destroyed if you jail break; your teacher will be stripped of standing in the college of magic if vouchsafed spell casting students jail break; your first born child will receive the spell caster’s sentence if they jail break; etc.).

• Appeal to the sanctity of law (if that is something that the spell caster cares about).

• Escalate the legal threats against the spell caster by compounding an original crime with a jail break (e.g., the punishment originally was 6 weeks in the clink and a small fine, now it is the amputation of your right hand).

• Make plea deals, so that publicly the spell caster is seen as punished, but the powers that be arrange to trade benefit for leniency in actual sentencing.

When a player describes an action and during that action it would make sense to use one of their class-features, describe that class feature in a vague manner and ask them if they want to use it. For example, if a player who doesn't know they are playing a barbarian says:

Player: I attack the bandits

GM: The moment you grab the hilt of your weapon, you feel an unnatural anger in your hearth. You feel you could crush these bandits if you give in to that anger, but you wonder if there is someone else who would be even more deserving of it. Would you like to embrace the anger or suppress it?

Player: I embrace the anger.

[... combat which is easier for the player than expected...]

GM: After the combat you feel tired, but your need for violence is satisfied. You don't think you will be capable of a violent outburst like that again until tomorrow.

Player: Ah, that was Barbarian Rage. Thanks!

Any spellcasters who need to prepare spells are difficult, because in order to use their spells they need to prepare them and in order to prepare them they need to know which spells they have. So they would be incredibly weak until they discovered their spells.

This doesn't affect Wizards, though, because Wizards need spellbooks to cast spells. So they would just need to look into it to learn what spells they know.

Divine spellcasters might start the game with a holy scripture to give them a hint what their religion is. That should give them a pretty good idea what spells they can use. If they try a spell they don't know, it should just fizzle.