if fail a caster level check on purify food/water do i know if its purified or not? pathfinder
say dc is 20 and i got a 16? do i know if the food/water is purified of any poisons?
I think you mean Detect Poison? PF&D doesn't mention any checks.
DP automatically tells you whether there's poison, no check. The check is to identify what poison is in there (if you detected any). Failure means "I know it's poisonous, but not what poison."