From the API it is possible to check how a specified trade execution is expected to change the account margin requirements for an account in real time. This is done by creating an Order object which has the IBApi.Order.WhatIf flag set to true. By default the whatif boolean in Order has a false value, but if set to True in an Order object with is passed to IBApi.EClient.placeOrder, instead of sending the order to a destination the IB server it will undergo a credit check for the expected post-trade margin requirement. The estimated post-trade margin requirement is returned to the IBApi.OrderState object.

This is equivalent to creating a order ticket in TWS, clicking "Preview", and viewing the information in the "Margin Impact" panel.