Is postgres installed?

Hi,

How I know whether Postgres is already installed or not on a machine(on
Linux and Windows)?
I found that pg_ctl --version could be used or I also tried to check into
the registry (for windows HKLM\SOFTWARE\PostgreSQL\Installations), however
with Windows Server 2003 pg_ctl is not recognized (probably not in the
path) and the registries are not written (nothing appears under
HKLM\SOFTWARE\)

Is there another check I can do?

Thanks
Christophe