powerhouse,
The title asked how to tell if Sendmail is on a server, but the body of the node asked how to tell which "mailing" system was present. This could be far from trivial to figure out:
Is the service running on the standard port - 25? You could try telneting into port 25 or netstat -an | grep LISTEN
If you are able to telnet into port 25 - does the banner identify iteself? Do you trust what it says? Can you type help to query further?
What program shows up in the process table? ps -ef and the like
Are there more than one mail program on the same system? Perhaps configured to work in conjunction with each other on different ports
Is there a start up script/config file for Sendmail
The list goes on and on. I would take jeffa's advice if you do not absolutely need to use Sendmail and make it as portable as possible.
Cheers - L~R