Look at what n_i said and open the manual for the system tables to find out what info you have there.
Another way depends on how you designed your database schema and the OS and FS.
On Mainframe, on VSAM datasets, with 1:1 partitioning, where all tables has it's own tablespace, the only thing you need to do is look at the extents for the dataset and see how many are in use. You can sort this in DSList.
I don't know how tablespaces are stored on Windows and AIX, but it may work there as well.