Hello,
It's not a definitive info but in combination with the previous points it can be usefull:
Telnet your host at the port number 25. Solaris default sendmail info prints "Sun" string after the release number and a plus sign (previous sysadmins could have changed the output text, but if changing something, the most common thing would be to remove all the info for security reasons, and not only Sun concerning info).
You can also check the configuration file's header comments for Sun strings. Again, the binaries can be standard binaries and the configuration file sun's file, but I don't think it's usual.
Bye,
jmiturbe