How to detect the server version?

You can get this via Firebird Service API. It does not work for Firebird Classic 1.0, so if you don't get an answer you'll know it's Firebird Classic 1.0 or InterBase Classic 6.0. Otherwise it returns a string like this:

LI-V2.0.0.12748 Firebird 2.0

or...

LI-V1.5.3.4870 Firebird 1.5

The use of API depends on programming language and connectivity library you use. Some might even not provide it. Those that do, call the isc_info_svc_server_version API.

If you use Firebird 2.1, you can also retrieve the engine version from a global context variable, like this:

SELECT rdb$get_context('SYSTEM', 'ENGINE_VERSION')
from rdb$database;