> I need to understand
> 1. if firebird is installed on a machine (win or linux)
> 2. where is installed
> 3. which version is installed
> How can I do it?

If the FB is running, you can attach to service manager as SYSDBA,
and you can get these informations using ServicesAPI:

Service Manager version: 2
Firebird Server version: WI-V2.1.1.17910 Firebird 2.1
Implementation: Firebird/x86/Windows NT
Root directory: C:\Program Files\Firebird\Firebird_2_1\
Lock file location: C:\Program Files\Firebird\Firebird_2_1\
Message file loc.: C:\Program Files\Firebird\Firebird_2_1\
Security database: C:\Program Files\Firebird\Firebird_2_1\security2.fdb

Ivan
http://www.volny.cz/iprenosil/interbase/