Finding the application behind ".net sqlclient data provider"
sp_who2 (MSDN) is always a good start, and you can query the sys.processes table or sys.dm_exec_connections (MSDN) DMV.

Alternatively something like Adam Machanic's sp_WhoIsActive can help find problem processes and queries. 

If you can't find what you're looking for directly in SQL Server, you'll be able to find things like the hostname/IP address and loginame that will help you track down the culprit.

Sample code for querying DMV/sys.processes (you'll have to chop it about if you want to do more):

Select spid,hostname,hostprocess,program_name,nt_username, blocked, waittime, waittype, loginame,cmd,spid,waittype,waittime,lastwaittype,cpu,physical_io,memusage,login_time,last_batch,open_tran,status,net_address, t.text 
from sys.sysprocesses sp 
--JOIN sys.dm_exec_connections con ON con.session_id = sp.sid
CROSS APPLY( select text from sys.dm_exec_sql_text(sp.sql_handle))t 
--where hostname
order by sp.spid