If you notice yourself surviving poison attacks unscathed, you know you must have a source of poison resistance. However, deliberately poisoning yourself is not a good way to identify this ring.
If you drop a ring in to a sink, and receive the message, "You smell rotten fruit" then you know the ring was poison resistance.
This is an excellent ring to discover early in the game, if you can identify it. You should wear uncursed rings in the hope that a randomly found one might be poison resistance.
If you know you have poison resistance through wearing this ring, you can then eat poisonous corpses until you gain the resistance as an intrinsic. There is a specific message, "You feel especially healthy." to indicate you received the intrinsic while currently protected by the extrinsic granted by the ring. This is useful to look out for as you can then dump this ring in favour of other, more useful ones. If you play at night, however, try to hang on to the ring in case a gremlin steals the intrinsic away.
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