How can I check whether SQL 6.5 or SQl 7.0 is installed without connecting to SQL and logging on?

A. You could check :-

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSSQLServer\MSSQLServer\CurrentVersion

However, in the case of a clustered server it may not be there. In which case you could check

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSSQLServer\Performance\Library

The value of this will either be sqlctr60.dll or sqlctr70.dll and the location won't change.

---