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Using the description from the DMG about Truth Serum (p. 257):

Truth Serum (Ingested). A creature subjected to this poison must succeed on a DC 11 Constitution saving throw or become poisoned for 1 hour. The poisoned creature can’t knowingly speak a lie, as if under the effect of a zone of truth spell.

Do others know if you have passed the saving throw when you ingest it (assuming they don't know the correct answer to the question they're asking)?

For example, a PC is being interrogated by a guard who suspects the PC's involvement in a crime that occurred. Giving the PC the opportunity to prove their innocence, the guard gives them a vial of Truth Serum to drink so they can ask some questions. Does the guard know whether the PC succeeds or fails on their saving throw?

I know that the Zone of Truth (PHB, p. 289) spell states:

You know whether each creature succeeds or fails on its saving throw.

But a poison seems like it would work differently from a spell – for example what if the character ingested the serum while the guard was not in the room? Are there any physical tells that the guard can identify to know whether the serum is working?

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The guard won't know if the player succeeded or not.

Like you note, the Truth Serum poison isn't precisely a Zone of Truth spell. The important line is this:

as if under the effect of a Zone of Truth spell.

This line tells us that the player isn't under a Zone of Truth, but that, for the purposes of lying, you should treat them as if they were.

Unless the guard has some other way of determining that the player has failed their save, they have no way of knowing if the player passed or failed. The guard would still be able to detect lies like normal using a skill check, so perhaps the guard could ask "is the serum working" and try to get the player to roll low on Deception.

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    \$\begingroup\$ Besides, even if the player was under the effects of a Zone of Truth, it'd be the serum that knew if they made the saving throw, not the guard. \$\endgroup\$ – Miniman Jun 29 at 22:58

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