In AD9523-1 register 0x22C, bits 2 and 3 indicate the status of the two reference inputs. The bits are set when the corresponding input is 'OK'. But what does OK actually mean? That transitions have been detected on the reference input pin? That a clock within a usable frequency range is present?
UPDATE: I've configured the STATUS0 output pin to show REF A status and probed the line with an oscilloscope. With no reference connected, I can see occasional 840ns pulses on STATUS0, perhaps 1 every 30 - 100us. Connecting a signal generator to the reference input, I see the pulses becoming more and more frequent as I increase the reference frequency. By 2.7MHz, STATUS0 is a square wave, 800ns on, 680ns off. Above 2.8MHz, STATUS0 stays high permanently. Note that this is with the AD9523-1 set up to expect a 10MHz reference input.