We have a server with several system administrator maintaining the server. Every administrator logs in using their own id and then su - to root.

Is it possible to see which user executed which commands (after they su - to root)?

No, su activities can't be audited back to an individual. You need a change control system. It doesn't need to be software, it can simply be a process. If you want an audit trail, enforce the use of sudo - prohibit su.

If you want an audit trail, enforce the use of sudo - prohibit su.
If you want an audit trail that includes commands users execute (when they su to other accounts), force Sudo but also force using a logging shell wrapper like Rootsh or Sudosh. The main difference between the two AFAIK is that Sudosh has session playback capabilities. If you want to expand on that make the wrapper log to syslog and log to a remote syslog host.

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You also may want to use a file integrity checker like Aide, Samhain or even tripwire to monitor changes. Top it off with a tool to monitor services for changes (like Monit). I have the most important configs under a revision system which makes it easy to check change info and revert back in case it gets fscked up.
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Thanks for your answers.

You can alias su to do something like this in /etc/profile.

alias su="su -p"

I'm guessing your seeing who did what based on bash_history or something... That'll keep it in their profiles.

man su
-m, -p, --preserve-environment
do not reset environment variables, and keep the same shell