The data sheet is a little bit silent on the details of the slave operation. But you migth get lucky that SPI_SpiIsBusBusy() also works for slave mode.
That's a really good idea!
the SCB SPI Slave header file (SPIS.h for me) has the following defined:
/* SPIS_SPI_STATUS_REG */ #define SPIS_SPI_STATUS_BUS_BUSY_POS (0u) /*  Bus busy - slave selected */ #define SPIS_SPI_STATUS_BUS_BUSY ((uint32) 0x01u)
Now the actual function doesn't exist, but this is definitely worth looking at. It seems it might be specific to EZSPI but this is a good direction to look.
Somerthing else I've noticed is that the pin-specific APIs seem to be exposed even if the SCB is "in charge" of the line, I might be able to query the pin register directly.