How can I know when SQL Full Text Index Population is finished?

When you execute some queries on your SQL Server, are you sure the catalog is being imported?

With this simple script you can know which is the Index status.

DECLARE @CatalogName VARCHAR(MAX)
SET     @CatalogName = 'TEST_FullIndex'

SELECT FULLTEXTCATALOGPROPERTY(@CatalogName,'ItemCount') as NumberOfItems, 
	   FULLTEXTCATALOGPROPERTY(@CatalogName,'ImportStatus') as ImportStatus,
	   DATEADD(ss, FULLTEXTCATALOGPROPERTY(@CatalogName,
                     'PopulateCompletionAge'), '1/1/1990') AS LastPopulated,
    (SELECT CASE FULLTEXTCATALOGPROPERTY(@CatalogName,'PopulateStatus')
        WHEN 0 THEN 'Idle'
        WHEN 1 THEN 'Full Population In Progress'
        WHEN 2 THEN 'Paused'
        WHEN 3 THEN 'Throttled'
        WHEN 4 THEN 'Recovering'
        WHEN 5 THEN 'Shutdown'
        WHEN 6 THEN 'Incremental Population In Progress'
        WHEN 7 THEN 'Building Index'
        WHEN 8 THEN 'Disk Full.  Paused'
        WHEN 9 THEN 'Change Tracking' END) AS Status
FROM sys.fulltext_catalogs AS cat

More information about the status or other property on Microsoft https://technet.microsoft.com/en-us/library/ms190370.aspx