Using OBJECTPROPERTY function and TableHasActiveFulltextIndex parameter. OBJECTPROPERTY(table_id, 'TableHasActiveFulltextIndex') will return a value of 1 when at least one column of a table is added for indexing.
CODE:

SELECT OBJECTPROPERTY(OBJECT_ID('MySchema.MyTable'), 'TableHasActiveFulltextIndex');
GO

REFERENCE: "Obtaining Full-Text Property Values using Transact-SQL Functions" http://msdn2.microsoft.com/ms142579.aspx

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Cristian Andrei Lefter, SQL Server MVP
MCT, MCSA, MCDBA, MCAD, MCSD .NET,
MCTS, MCITP - Database Administrator SQL Server 2005
http://sqlserver.ro