Try look at http://www.sqlservercentral.com/articles/tempdb+utilization/65149/

The following will give you the amount of free space within tempdb:

SELECT sum(unallocated_extent_page_count) [Free_Pages],

(sum(unallocated_extent_page_count)*1.0/128) [Free_Space_MB]

FROM sys.dm_db_file_space_usage

This script will show you which SPID takes up the most space in tempdb:

SELECT top 1000

s.host_name, su.[session_id], d.name [DBName], su.[database_id],

su.[user_objects_alloc_page_count] [Usr_Pg_Alloc], su.[user_objects_dealloc_page_count] [Usr_Pg_DeAlloc],

su.[internal_objects_alloc_page_count] [Int_Pg_Alloc], su.[internal_objects_dealloc_page_count] [Int_Pg_DeAlloc],

(su.[user_objects_alloc_page_count]*1.0/128) [Usr_Alloc_MB], (su.[user_objects_dealloc_page_count]*1.0/128)

[Usr_DeAlloc_MB],

(su.[internal_objects_alloc_page_count]*1.0/128) [Int_Alloc_MB], (su.[internal_objects_dealloc_page_count]*1.0/128)

[Int_DeAlloc_MB]

FROM [sys].[dm_db_session_space_usage] su

inner join sys.databases d on su.database_id = d.database_id

inner join sys.dm_exec_sessions s on su.session_id = s.session_id

where (su.user_objects_alloc_page_count > 0 or

su.internal_objects_alloc_page_count > 0)

order by case when su.user_objects_alloc_page_count > su.internal_objects_alloc_page_count then

su.user_objects_alloc_page_count else su.internal_objects_alloc_page_count end desc

Hope that helps, any other questions just let me know.