Hello Kuntesh,
Based on the support ticket I post the answer in here as well.
You are correct that this will be problem to troubleshoot as the way how the payement gateways work within Kentico is that We are using POST method to post data in following parameters to Authorize .NET provider:
x_test_request
x_login
x_tran_key
x_version
x_relay_response
x_delim_data
x_delim_char
x_encap_char
x_description
x_invoice_num
x_exp_date
x_card_num
x_card_code
x_method
x_type
x_amount
x_currency_code
And there is only a response code returned from the gateway back to application.
In this case you could implement custom payment gateway, that is actually the same as the default authorize.Net gateway:
https://docs.kentico.com/display/K82/Creating+a+custom+payment+gateway
Than you can add some EventLogProvider.LogEvent() method to log certain details that could help you find the problem.
Other option is to use traffic monitoring tool (e.g. Fiddler: http://www.telerik.com/fiddler) to track posts and responses to see if there are any issues with data posted.