In a symphony orchestra almost every every player is able to fine-tune the pitch of notes while playing. The absolute exceptions are the piano (if used), harp, open notes on stringed instruments, and tuned percussion. The oboeist gets to maintain a stable note (A) for everyone else to tune to. String players tune open notes. Wind players including brass players know what concert A 'feels' like when it is centred.
On brass instruments the valve tuning slides generally don't need much adjustment, especially if players are always playing at the same pitch in an air-conditioned concert hall. If it is not quite right the player would be able to 'lip' it into tune. A good trombonist knows where the positions of the instrument are and will be able to get A reliably. If the tuning slide is not quite right a good player will use lip and slide to adjust.
When I played in junior orchestras many years ago, some conductors used the oboe to give a B-flat for the brass, and some expected all players to tune to A. Some would check by asking all brass players to play (concert) B-flat. I never came across a conductor who would do anything special to help the player of the G bass trombone.