A good starting point is the sp_who and sp_who2 system stored procedures (though sp_who2 isn't officially documented). These take information from a variety of system tables to include sysprocesses and producs a readable set of information.
You may also take a look at DBCC INPUTBUFFER() as well as fn_get_sql in SP3 in order to confirm a SPID you suspect is actually the one you need to kill. More info on fn_get_sql:
Books Online - fn_get_sql
K. Brian Kelley
@kbriankelley