Someone has told me once that as long as an item is sold and doesn't involve magic, then a character should have access to what it is. I can't recall verbatim what they said, but that's the gist of it. Context being that a gm in pfs didn't know how to handle telling the players that they had alchemists fire or not.
I come to wonder if there are any rules for knowledge of items. Tanglefoot bags, acid flasks, etc. or even perhaps if one doesn't have martial weapon prof. Then maybe a cutlass or a longsword are just "sword"?
Anyone have insight on how to deal with making some sort of knowledge check in a situation there aren't rules spelled out about doing this sort of thing?
No rules per se, but here are some guidelines regarding Firearms from Ultimate Combat:
Firearms in Your Campaign
Firearms and gunslingers are not for every campaign, and even if you are excited about introducing firearms into your campaign, you should still make a decision about how commonplace they are. The following are broad categories of firearm rarity and the rules that govern them. Pathfinder's world of Golarion uses the rules for emerging guns, which is also the default category of gun rarity detailed in this Pathfinder RPG supplement.
No Guns: If you do not want guns in your campaign, simply don't allow the rules that follow. The Pathfinder Roleplaying Game plays perfectly well without them.
Very Rare Guns: Early firearms are rare; advanced firearms, the gunslinger class, the Amateur Gunslinger feat, and archetypes that use the firearm rules do not exist in this type of campaign. Firearms are treated more like magic items—things of wonder and mystery—rather than like things that are mass-produced. Few know the strange secrets of firearm creation. Only NPCs can take the Gunsmithing feat.
Emerging Guns: Firearms become more common. They are mass-produced by small guilds, lone gunsmiths, dwarven clans, or maybe even a nation or two—the secret is slipping out, and the occasional rare adventurer uses guns. The baseline gunslinger rules and the prices for ammunition given in this chapter are for this type of campaign. Early firearms are available, but are relatively rare. Adventurers who want to use guns must take the Craft Firearms feat just to make them feasible weapons. Advanced firearms may exist, but only as rare and wondrous items—the stuff of high-level treasure troves.
Commonplace Guns: While still expensive and tricky to wield, early firearms are readily available. Instead of requiring the Exotic Weapon Proficiency feat, all firearms are martial weapons. Early firearms and their ammunition cost 25% of the amounts listed in this book, but advanced firearms and their ammunition are still rare and cost the full price to purchase or craft.
Guns Everywhere: Guns are commonplace. Early firearms are seen as antiques, and advanced firearms are widespread. Firearms are simple weapons, and early firearms, advanced guns, and their ammunition are bought or crafted for 10% of the cost listed in this chapter. The gunslinger loses the gunsmith class feature and instead gains the gun training class feature at 1st level.
If your campaign were using the "Very Rare Guns" or "Emerging Guns" guidelines, I would say it would be perfectly reasonable to require Knowledge: Mechanical to figure out what this weird stick did (trigger looks like a crossbow's, so it's probably a weapon of some kind…). Identify—the spell—specifically says it aids in the detection and understanding of magical items though, so it would not be appropriate for identifying a mundane item.
For chemical, alchemical, biological or geological knowledge, you could house-rule their rarity and scarcity in the same way. Not all countries discovered saltpetre or quinine or the difference between acids and bases at the same time, after all.
If you're playing in PFS, I don't think that's a call your GM can make though. In PFS, I believe the assumption is that the people of Golarion have ready access to Alchemist's Fire. It's in the CRB and is not listed as being exotic or rare in any way.