Not necessarily. If it was something that made you ill after ingestion, and yet passed, it isn't likely a poisonous compound. True poisons need to be cleansed or countered.

[not that he can tell for sure, just on description, but Law is capable of removing poisons from himself or others should they be ingested. So, eating a poisonous fish isn't as dangerous for his tests as it is others]

Can't tell that from here, just with my senses. I can only sense their basic size and power level. They may be dangerous or not, who can say?