You can't. Sometimes, you can't even tell by reading the docs! There's a couple of signs, however.

• Does the module use %^H or $^H? If so, almost definitely.
• Does the module use %^H or $^H? If not, it could still be.
• Does the module (or a parent) have an unimport method? If so, it's almost certainly a pragma.
• Does the module (or a parent) have an unimport method? If not, it's definitely not a pragma.
• Is the module name lowercase? If so, it has a higher chance of being a pragma.
• Is the module name lowercase? If not, it has a lower chance of being a pragma.
• Is the module a core module with a lowercase name? If so, then it's a pragma.
• Is the module a core module? If not, it unlikely to be a pragma.

Based on the last two points, you'll be almost guaranteed to be correct if you simply check the module name against the list of core modules that are pragmas.

Update: My post and the list Mutant posted discuss lexically scoped pragmas. The list I posted above define the term "pragma" more loosely. I'm not sure which one you want.