In TWS you can add columns to your watchlist or Portfolio tabs pertaining to ETFs. There are several "NAV" columns that you can add that will provide information on an ETFs Net Asset Value calculated based on the prices of the ETFs underlying securities.
To add ETF columns to the Quote Monitor or one of the tabs listed above, hold your mouse over an existing Market Data column heading and click the green "+" sign to insert column. From the displayed list select ETFs. Here you will find "NAV" columns such as NAV Close, NAV Last, NAV Prior Close, etc.
For more information on the ETF Columns, please visit the TWS Users' Guide.