You obtain the data from the MON$ATTACHMENTS table using a simple SELECT query. Refreshing the query that reads the MON$ATTACHMENTS table is done by comiting the transaction within which the query was issued and then repeating the whole SELECT operation again.This will ensure you see the latest info from the MON$ATTACHMENTS table.
However, it sounds like you want the server process to more quickly conclude that other connections have been severed. That is another problem altogether. If the server has not yet concluded that some attachment has been severed, refreshing the query (as described above) will NOT force the server to look again at the current attachments. It does that on its own time. I believe I recently saw someone post the details on how to change your configuration so that the server takes less time to detect severed attachments. Look back in this thread to see if it was there. [Non-text portions of this message have been removed]