What is the best way to know if an element is " empyt " inside a mapping ?

For example

mapping ( address => uint) balances;

If the user already used the contract there will be an address => "foo" value but if not the mapping still exist it just gives back 0x00000 or something as value.So it's never really empty.

Is there a clean way to check if the value is 0x0000 or something an user inputted for that address?( other than if balances[address] == 0x0000 )



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