At the start of season 1, episode 19 - Solitudes of Stargate SG1, before the title credits, there is an incoming wormhole which for some reason destablizes and disconnects before the entire team gets through. However, before the connection is initially established, the following happens:
stargate is currently locking chevrons for an incoming connection
General Hammond: There's no one scheduled to come back for 24 hours, who is it?
Walter Harriman: It's SG-1's remote access code, Sir.
This occurs more frequently much later in the series but I always passed that off as Earth's military technology having improved over the duration of the show, however this is really early on and they haven't gained anything like that technology by this point.
How does SGC know who is incoming before the stargate connects?