There are many excellent suggestions here. I would like to expand on one of them, though.

Some systems/settings have spells or abilities which can directly counter Detect Lies (or the equivalent). I recommend, if you end up using this in your campaign, making such counter-tactics obvious to the players, so they know exactly what's going on. Telling the mage "sorry, that doesn't work" is no fun for anyone.

As a specific example, Corrupted Words is a Terrestrial Circle Sorcery in Exalted. The caster chooses a subject while casting the spell, and until the spell is broken, whenever the target of the spell (which could potentially be the caster himself!) even attempts to communicate about that subject -- whether voluntarily, under duress of mundane interrogation, or under the effects of magic -- they begin vomiting up maggots, and become incapable of communicating what they attempted to convey.

This does several things:

1. It is immediately obvious that the subject is under some kind of geas to prevent them from talking. Even if the characters (or players) are unfamiliar with Corrupted Words, the fact that the subject starts vomiting maggots when they capitulate should be a clue.
2. If the character recognize the effects, they know the "other guys" have at least one sorcerer on their side. This gives them some information despite the failed interrogation.
3. If the party contains a sorcerer, that character ought to know one of Emerald, Sapphire, or Adamantine Coutnermagic, any of which would remove the effects of Corrupted Words and allow the interrogation to continue. (Alternatively, a higher-level necromancer in the party might know Onyx or Obsidian Countermagic.)

There are many excellent suggestions here. I would like to expand on one of them, though.

Some systems/settings have spells or abilities which can directly counter Detect Lies (or the equivalent). I recommend, if you end up using this in your campaign, making such counter-tactics obvious to the players, so they know exactly what's going on. Telling the mage "sorry, that doesn't work" is no fun for anyone.

As a specific example, Corrupted Words is a Terrestrial Circle Sorcery in Exalted. The caster chooses a subject while casting the spell, and until the spell is broken, whenever the target of the spell (which could potentially be the caster himself!) even attempts to communicate about that subject -- whether voluntarily, under duress of mundane interrogation, or under the effects of magic -- they begin vomiting up maggots, and become incapable of communicating what they attempted to convey.

This does several things:

1. It is immediately obvious that the person is under some kind of geas to prevent them from talking. Even if the characters (or players) are unfamiliar with Corrupted Words, the fact that the person they're interrogating starts vomiting maggots when they capitulate should be a clue.
2. If the character recognize the effects, they know the "other guys" have at least one sorcerer on their side. This gives them some information despite the failed interrogation.
3. If the party contains a sorcerer, that character ought to know one of Emerald, Sapphire, or Adamantine Coutnermagic, any of which would remove the effects of Corrupted Words and allow the interrogation to continue. (Alternatively, a higher-level necromancer in the party might know Onyx or Obsidian Countermagic.)