As an action, you can create a vial of liquid. A creature can inject the serum as an action. The injected creature must make a Will saving throw.
On a failed save, a creature can’t speak a deliberate lie for the duration. You know whether the creature succeeds or fails on its saving throw. An affected creature is aware of the effect and can thus avoid answering questions to which it would normally respond with a lie. Such a creature can be evasive in its answers as long as it remains within the boundaries of the truth.