how to check when was the successful DBCC CheckDB run in sql server

its simple.

dbcc dbinfo ('db1') with tableresults
and then check for "dbi_dbcclastknowngood" field.

you can also get a host of other fields and values from dbinfo command.

Also the below command gets you the same answer:

dbcc page (dbname,1,9,3) with tableresults