In a SQL Proc you may want to check for transactions using the @@TRANCOUNT system variable.

The query will return 0 for none, or a positive number (only 1 is valid at this time). If no database is open or if the database is in an invalid state -1 will be returned to represent an error condition.

SELECT @@TRANCOUNT; 

A stored procedure capable of handling an exception in a TSQL Proc that contains transactions should also check the TRANCOUNT and rollback if it is found to be greater than 0.

BEGIN TRY
{ sql_statement | statement_block }
END TRY
BEGIN CATCH
-- Do we have a transaction to rollback?
IF @@TRANCOUNT > 0
BEGIN
ROLLBACK TRANSACTION;
END
END CATCH