Method #1

The normal way would be to look at the errorlog, but this is not always convenient or even possible. From a SQL session you find out the server startup time to within a few seconds using:

select "Server Start Time" = crdate

from master..sysdatabases

where name = "tempdb"

Method #2

Another useful query is:

select * from sysengines

which gives the address and port number at which the server is listening.