Back story I have an alchemist in my group. They recently faced zombies who had gas canisters which started releasing a inhaled poison when they got up off the ground, this gas attack would go for 3 rounds after getting up and affected every adjacent square. The alchemist wanted to identify the poison being released. So I gave him a chance to identify it based on its colour and apparent effects on the others visually. He failed the check. After combat he wants to take out his portable alchemical lab to examine the container. Question I cant find any rules for identifying poison, so should I just use the similar rules for potions?
There are no rules for nonmagically identifying a poison.
In general, the rules for poison in 3.x systems aren't very well fleshed out. There is very little information on creating and balancing new poisons, or how purchasing the (presumably illegal) poison ingredients might work, or (as you note), identifying them nonmagically.
There is one RAW way of identifying poisons that I can find: the spell detect poison. With this spell, you can make a DC 20 Wisdom or Craft(alchemy) check to identify what specific poison you're looking at.
However, it makes sense that an alchemist skilled in the creation of poisons shoudl be able to identify them nonmagically. The houserule that I use is to have poisons work like potions in this way:
Identifying Potions: In addition to the standard methods of identification, PCs can sample from each container they find to attempt to determine the nature of the liquid inside with a Perception check. The DC of this check is equal to 15 + the spell level of the potion (although this DC might be higher for rare or unusual potions).
For the spell level of the poison, I use the Fort save DC (which is also the Craft DC), minus 10, divided by 2. This gives a number something like a reasonable spell level for this purpose. For example, Burnt Othur Fumes would have a DC of 19.