You can monitor your margin values in real time using your TWS/Mosaic Account Window by looking in the Margin Requirements and Available for Trading sections. In the IBKR Mobile open the Account screen and tap on the Margins or Funds tabs. If you would like to see your margin information broken down by position as a daily snap shot you can create a Margin Report in Client Portal. More information on the Margin Reports can be found in our Users' Guide.
To see how the account's real time margin requirements would change when previewing an opening or a closing order you can use the Check Margin Impact feature in TWS or in Check Margin Impact in Mosaic. In the IBKR Mobile trader tap Preview in the bottom of the Order Ticket.