debugging

Do you have a specific scenario you need debugging or are you just asking in general?

if i am getting an error in interface then how to  find interface from site 

There are a few things you can do:

1. reproduce the error - an Appian interface has rule inputs (parameters if you will). If you know what those values are for the specific instance where you're experiencing the error you can recreate the error by loading up the interface in the Designer tool and providing those values. 
2. You can look at the Appian logs, these will have information in them pertaining to the error you're seeing, you just need to find the right entry in the logs correlating with the error

if i am getting the error in logic which is present in interface then how to debug

Typically Appian will give you an error message that is usually (!) descriptive of what the root cause is. Do you have a specific error message you need help with?

Try to execute the same logic in an expression rule by passing the same input values to find out the the output of logic is according to your expectation or not.