Proof against poison is a prefix. And the game does tell you which shards are prefixes and which are suffixes, it just does so subtly. When you hover your mouse over the shard icon in-game, if the name of the shard says shard of xxxxxxx then it is a suffix. If it says xxxxxxxx shard, then it is a prefix. The placement of the shard name relative the word shard, before or after, tells the difference. This always tells you which it is and it can be used before you craft it by looking at it in the crafting machine.