After powering up the radio, write 0x20 to XTAL_CTRL_ADR register. Then use a high precision frequency counter (HP53181A or eq.) to measure crystal PPM via the XOUT pin:
// XTAL_CTRL_ADR=0x20 - drives 12MHz onto XOUT pin
Expect the measurement to be within the 11,999,640Hz to 12,000,360Hz range. If the measurement falls outside the expected range verify that the crystal's load capacitance is 10pF.