How to sense JTAG/SWD connected?

Practically, like this on Cortex M7:

bool is_debugger_enabled()
{
 // NOTE: After disconnecting debugger, do power cycle because
 // software reset does not seem to clear DHCSR.C_DEBUGEN!
 // DHCSR and debugger behavior are described in ARM v7-M Architecture Reference Manual
 return (CoreDebug->DHCSR & CoreDebug_DHCSR_C_DEBUGEN_Msk); // bit 0: DHCSR.C_DEBUGEN
}