The RFdc driver API can be used to get the decimation rate set in the IP core using the following code.
// Get Decimation factor for Tile0, DDC Block1
int Tile = 0;
u32 Block = 1;
u32 Decimation_Factor;
if( XRFdc_GetDecimationFactor (ptr, Tile, Block, &DecimationFactor) == XST_SUCCESS) {
xil_printf("ADC Tile%1d,%1d Decimation Factor is: %d", Tile, Block, Decimation_Factor);
}