Any estimation is footnoted. You can access it on "View Footnotes" link in data query result.

For an example:

Selected classification: SITC Rev.1
Selected commodities: 33101 (Crude petroleum)
Selected reporters: All
Selected years: 2006
Selected partners: World
Selected trade flows: Import

Result is http://comtrade.un.org/db/dqBasicQueryResults.aspx?cc=33101&px=S1&y=2006&p=0&rg=1&so=1001&qt=n
and Footnote is http://comtrade.un.org/db/dqBasicQueryFnResults.aspx?cc=33101&px=S1&y=2006&p=0&rg=1&so=1001&qt=n

However, we understand that sometimes it is not clearly marked (noted for further improvement)