It's in PANS ATM DOC 4444.
Para 12.4.1 Identification of aircraft.
I'm a Comms instructor at BGS and a couple of students had the question in their ATPL IFR exam. I found the TX in 4444 but cannot find the expected pilot action and response.
Of the 4 options given these two "appear" to be relevant:
Report heading.
Press IDENT on the transponder and report heading.