select * from sys.dm_db_file_space_usage

We can use the sys.dm_db_file_space_usage dynamic management view to monitor the spaced used by tempdb for storing user/internal objects and version store.

sys.dm_db_file_space_usage has following columns:

1. database_id – will always be 2, database id of tempdb.

2. unallocated_extent_page_count – total number of free extents in the database file

3. version_store_reserved_page_count – pages allocated for version store

4. user_object_reserved_page_count – pages allocated for user objects, these are the permanent object you create explicitly in tempdb, system tables, temporary tables, table variables…

5. internal_object_reserved_page_count – pages allocated for internal objects

for details about other columns returned refer BOL.