For getting the successful cPanel/WHM logins, using the following commands.

For getting the successful cPanel logins.

cat /usr/local/cpanel/logs/session_log | grep "myuser" | grep "NEW .*app=cpaneld" 
# "myuser" is cPanel username

xx.xx.xx.xx [09/11/2015:13:17:40 -0000] NEW myuser:bOMyuserz8hKvThis1XUgnISnh4AWMMXSampleKgHE3lString1JpnOSiK5a45t3 address=122.165.84.98,app=cpaneld,creator=myuser,method=handle_form_login,path=form,possessed=0

For gettign successful WHM logins

cat /usr/local/cpanel/logs/session_log | grep "root" | grep "NEW .*app=whostmgrd" 
# Use required WHM user instead of "root"

xx.xx.xx.xx [09/05/2015:07:22:39 -0000] NEW root:KGJsPXy_h243av2XchbPGBajfsSDj0eRz4ryHIj_tGZtYODFSZVb5s4rkdx20LJd address=xx.xx.xx.xx,app=whostmgrd,creator=root,method=handle_form_login,path=form,possessed=0