A normal profiler trace should do it, or look in the default trace (should be caught under Audit DBCC Event)
Someone or something is running either CheckDB or CheckTable if you see that in running sessions, checkDB does not ever run by itself and nothing inside SQL will trigger it.
p.s. You need to do something about not having a regular checkDB job.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability