Greetings,
I believe the easiest way is to look at the version of the server.
You can query it in the server as follows:
SELECT @@version
GO
However, this approach only works if you remember what was there before.
If you don´t, check the log as suggested by our piers and look at the boot sequences. The server version always appears in there, and if the log has not been archived, you should see different versions of the server across boots. In my experience, a log will contain at least one or two boot sequences.
If the log does not, then you have to go to older archived logs. That is SOP. Standard Operating Procedure. Like the .cfg file which gets backed up with each parameter change... E.g. .001, .002, etc.
I hope this helps.
Regards,
Jean-Pierre