According to the description of Bag Of Holding
If a bag of holding is overloaded, or if sharp objects pierce it (from inside or outside), the bag immediately ruptures and is ruined, and all contents are lost forever.
Does this mean that it is possible to place a doomsday device or an evil magical artifact in the bag of holding before intentionally rupturing it to destroy those things?
As written, yes.
But in practice, artifacts often break the rules and surviving this kind of mishap would be something I expect if the plot hinges on the artifact’s existence and the impossibility of destroying it. The Lord of the Rings would have made a pretty poor story if the One Ring could have been destroyed, and so too might your campaign, so don’t be surprised if the GM nixes it.
This GM would allow a discern location effect or similar effect to locate a ripped bag's contents individually despite those contents being—supposedly—lost forever, but this player wouldn't storm out of another GM's campaign if that GM ruled that was impossible. Rest assured, though, that in such a GM's campaign, this player would—could his PC afford them—invest in a few extra bags of holding for disposing of artifacts of great evil and corpses of defeated villains!
To be clear, this GM would allow the discern location effect to work because descriptions like this often appear to this reader to be written in isolation, without regard to the rest of the game, forcing the GM to make decisions about the description's actual meaning. That is, while this GM totally approves of PCs spending 2,500 gp on a bag of holding so that it can be employed as single-use garbage disposal, this GM interprets the bag's description to mean that if the bag is stuffed too full or poked with a longsword or whatever then the bag's contents are forever lost if the bag of holding is what's being relied on to retrieve those contents.
Lots of things in Pathfinder and its antecedent D&D 3.5e make (some would say grandiose) claims that seem to consider nothing beyond their own descriptions. For example, the spell contagion says that the spell's subject (if the subject fails a saving throw and the caster overcomes the subject's SR) contracts a disease, but I've never heard of a GM ruling that a subject that is immune to disease—like the typical level 3 paladin—contracts a disease anyway because the contagion spell says the subject contracts a disease! Similarly, a level 20 fighter will laugh at the "great ability in unarmed combat" that's conferred upon her by a monk's robe because that ability is like that of a level 5 monk—seriously, great ability in unarmed combat is by its nature a relative and context-dependent description.
The game is too vast to list every exception—the contagion spell won't bother saying that the subject contracts the disease unless the subject's immune to diseases, for instance, nor will the monk's robe say that it confers great ability in unarmed combat unless you're already aces at the punching—, leaving it up to the reader to determine if each absolute statement is an actual absolute or if exceptions still apply.1
Thus even a ripped bag of holding's description of its contents being lost forever needn't be an airtight absolute. The spell discern location really can find any object if the caster's first touched the object, and the bag's description of its ability to lose things seems to this reader as specific as the spell's description of its ability to find things! This GM would go ahead and make the call and let the discern location effect work, lost forever usually being terrible for storytelling anyway.
So ask your GM how absolute lost forever is… and be sure to mention the spell discern location.
1 Usually determining which exceptions apply is done via determining which effect is more specific (i.e. the specific-beats-general rule that's the crux of exception-based game design). However, in many cases—like this one—determining what exactly is more specific is an exercise in futility.
