How to determin if current computer has functioning sql sever(s)installed?

I mean, sql server version wise, 2000 and above, and computer OS
primary XP (Pro or Home), and to determine it/them from DOS command
line programmatically.

a) Windows' registry has information on installed sql server instance,
however, one issue with that is expired sql server instance, which is
still registered, and I manually confirmed it with the Add/Remove
Software tool.

b) The net start command is handy, but then, what is an instance
accidentally failed to start but, say, 98% time is reliable.

And programmatically Option a) is easier to work with than Option b).
What's your thought?

Thanks.