Often there are loot chests in obvious places, but more often you find them in some obscure corner where you don’t think to look. There are several different levels of chests which isn’t that important since you want to find all of them. The higher level chests have a better chance of spawning better weapons though.
The way you find most loot is you look around. There is an incredible amount of loot to be found in Borderlands 2, its rare that a box doesn’t contain some money or ammo. Be sure to make a mental note of the location of the chests so when you play in the same area again you know where to look.
Gun Loot Chests on Raised Platforms
Most often you have to complete a small jumping puzzle to get to a loot chest. You can jump very high in Borderlands 2 so be sure to try every possible angle should you find it difficult to reach a chest. Always look for a higher ledge to get a running jump start for easiest access to the loot chest.
Gun Loot Chests on Top of Buildings
A lot of times there is loot on top of buildings. You can find various creative ways of getting to the top, I haven’t encountered any invisible walls in Borderlands 2 yet. So you can jump on barrels, fences etc to gain access to roofs.
Guns in Toilets?
I have found many guns in the toilet boxes (similar to those you find on construction sites). Let the goo wash out and then a few guns should pop out.