Oh, sorry.
Well if you go to the control panel itself, on the login screen lots of times it says the version in the page title in your browser.
Also some times it may say so on the login page itself - for example the default theme for plesk 9.5 has "Log in to Parallels Plesk Panel 9.5" in text on the page, and in the browser page title.
You can also look at the page source of the control panel, sometimes it gives you hints
<link rel="shortcut icon" href="/favicon.ico">
<title>Parallels Plesk Panel 9.5.4</title>
<script language="javascript" type="text/javascript" src="/javascript/frameset.js?plesk_version=psa-9.5.4-95110630.15"></script>
or
<title>Parallels Plesk Panel 11.0.9</title>
<link rel="shortcut icon" href="/favicon.ico">
<script src="/javascript/frameset.js?plesk_version=psa-11.0.9-110120608.16"></script>
Some servers have a response header set that you can see in fiddler or most browser inspectors (hit F12 in your browser and look at the network tab) that may indicate the OS version.
Typically though if you are a customer there you should know what the OS type is ahead of time. The most common version of Plesk (just by looking at the forums) is linux, so you're more likely to be there than on windows, but you can easily tell the difference based on what features are available to you in the control panel once you login.
For example windows may offer you things like MS SQL, ODBC DSN connections, .Net Framework management, app pools, or Frontpage to name a few while Linux wont offer that.