Conditional Data Validation

I need to perform conditional data validation but am not sure the best way to go about it.  I have performed more complex validation, like verifying passwords match with a new account registration, but I need to perform validation on fields that are dependent on another field.  For example, if a field is checked (true) then validate some additional fields, but if it is not checked (false) then skip the validation.  Seems pretty simple but I haven't been able to come up with a good solution.  Any help would be greatly appreciated.

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