If you are using the Polkadot.js UI or the Chrome extension, you will see your balance in the accounts section.

You can also enter your address in the block explorers Polkascan or Subscan to look up your balance.

Polkascan

Make sure the Polkadot network is selected on top, then enter your address and hit enter: 

This will bring up your balance on the next screen:

Subscan

Select the Polkadot network and enter your address. Then hit Search.

This will bring up your DOT balance on the next screen: 

It doesn't matter which block explorer you use. Both access the same information.

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