It is also necessary to consider the type of account (POP, IMAP, or Exchange), and whether the folders are local (on the computer) or on the server.
In the case of POP accounts, the folders will be on the computer and/or iPad. They will not be synced, only a record of emails saved on those devices.
In the case of IMAP or Exchange, the folder may or may not be local. Only the settings on the device will tell you if. If they are local, then like POP accounts, they will only reflect emails sent/received on that device or computer. If they are server based they should be the same across all devices and/or computers.
If the folders are server based on the iPad (default for all accounts except POP), then you can scroll to the bottom of the email list and see a More option (if it doesn't just automatically start loading more).
To see how email folders are configured on the iPad, open the account in Settings > Mail, Contacts, Calendars and dig down to the Advanced options (tap on stuff till you find it).
You are on your own for computer clients. Each is different, but there is probably a help option to show the way.