How do I know what manufacturer's parts are on a gun?
Infographics of various weapon parts:
- SMGs
- Rocket Launchers
- Sniper Rifles
- Shotguns
- Pistols
- Assault Rifles
Source and higher-resolution version of images from Reddit user Zikel's "Weapon parts infographic" series of posts at /r/Borderlands:
- Weapon parts infographic [#1] SMGs
- Weapon parts infographic [#2] Rocket launchers
- Weapon parts infographic [#3] Sniper rifles
- Weapon parts infographic [#4] Shotguns
- Weapon parts infographic [#5] Pistols
- Weapon parts infographic [#6] Assault rifles
It seems the only way to check in-game is to visually recognize the parts; some are more obvious than others, for example Vladof Sniper and Pistol barrels are always double, rotating barrels.
You can check the manufacturers' parts directly using Gibbed's Borderlands 2 Save Editor (link is to the most recent version as of now, it's actively updated and I don't think there's a single page to always get the latest version).
Basically you just load up your character's save file, go to "bank" or "backpack" and you can check out your stored/current gear and what stats they have. You can also of course edit them, but that's not required (and make sure to save your game before doing any editing yada yada yada).
For example here's a Maliwan legendary Norfleet clearly showing parts from multiple manufacturers:
Different parts have different manufacturer options so there's really no easy way to tell what's what without looking through this tool (or recognizing the parts after becoming familiar with them via this tool).
Note that the tool rapidly becomes out of date (after each patch) and may not work properly until a new version is released after a patch. DLCs tend to break it since they introduce new guns.
If you're not sure how to use the editor, guides for the current version can usually be found in the Se7enSins forums for Borderlands 2 Modding. A stickied thread on there right now has a compilation of various related threads.
The guns are always made by just one company.
Vladof guns (and therefore also Vladof barrels) have the highest firerates. Tediore always have the explosive reload. Bandit guns (the ones with the horrible spelling) have the largest magazines. Maliwan guns are always elemental (and in general very good). Jakobs guns are old-style high damage (the revolvers that fire as fast as you can pull the trigger). Hyperion guns are more accurate the longer the trigger is held. Dahl guns aways fire in bursts when aiming down sights. Torgue guns always deal explosive damage.
This does not mean that Vladof guns can't have elemental effects for example. A manufacturers gun will always have the classic manufacturer effect though.
Additional info: Guns name prefixes (such as "Perma-Sharp SMG") are dependent on certain gun accessorys. These are effects/modifiers like 50% more melee damage. Here is a complete list: Strength Gamer Guide: 'Borderlands 2' Accessory Prefixes List
Hope I managed to help, all based on personal experience as a level 50 (as of 6 days ago^^).