If you are using the Polkadot-JS UI or the Polkadot extension, you can see your balance in the accounts section. 

Please keep in mind that the extension is an account manager, not a wallet, and it doesn't display the account balance. Your extension accounts will be visible in the Accounts page on Polkadot-JS UI, along with their balance, with a type "injected".

The balance displayed next to your account is its Total balance. You can click on the little arrow on the right to expand the view and see a breakdown of your account's balance (transferable, locked, reserved etc). 

NOTE: Polkadot-JS UI displays only the accounts' balance, not their transaction history. For that, you'll need to view your account on a block explorer.

Block explorers

You can also enter your address in a block explorer to look up your balance and transaction history. Clicking on the icons of the supported block explorers next to your account will take you to your account on that block explorer.

Regardless of which block explorer you use, entering your address in the search field will bring up the details of your account. The displayed information depends on the specific block explorer.

NOTE: All block explorers should display the same data for your account. If you find that some of it is not correct on one, try searching for your account on a different block explorer, as it may be that the particular block explorer is having issues.