Magento: which folder can I see my installed extensions?

typically in [MagentoFolder]/app/code/community/[SomeCompanyName]/[SomePackageName]

(otherwise, instead of "community" it could be "local").

For the location of Magento itself on your server, that depends on where you installed it! :)

The extension can either be in one of the following directory:

[YourMagentoFolder]/app/code/local/

OR,

[YourMagentoFolder]/app/code/community/

Here is the list of folder structure for all the files that can be in a Magento extension:- Magento Folder Structure