Checking status of an ORACLE instance

I have written a service that runs on an
Oracle server (7.3.3.0.0 for Windows NT)

The service connects to the database, and
does secret things in the background.

My question is:
How can my service check the status of
the Oracle instance it tries to connect to?
(Is the instance started or stopped)

Thanks in advance.

Michael Andersen, B.Sc. EE
SFK Meat Systems
Denmark