1st check which MTA is running; you can have only one...
root@blah~] netstat -tanp|grep 25
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN 1950/master
tcp 0 0 ::1:25 :::* LISTEN 1950/master
shows 'master' which is the key part of postfix.
sendmail should show up as 'sendmail'
For logs, try /var/log/maillog