EASA learning objective 022.02.06.01.09 states: Define the term ‘unreliable airspeed’ and describe the means by which it can be recognised such as: different airspeed indications between ASIs; unexpected aircraft behaviour; buffeting; aircraft systems warning; aircraft attitude.

EASA learning objective 022.02.06.01.10 states: Describe the appropriate procedures available to the pilot in the event of unreliable airspeed indications: combination of a pitch attitude and power setting; ambient wind noise inside the aircraft; use of GPS speed indications and the associated limitations.

 EASA CS 25 states that the difference between IAS/CAS indications must not exceed 3% or 5 knots whichever is the greater. A persistent discrepancy of more than 5kts between ASI's will normally trigger a caution message. There will be an 'Unreliable Airspeed' procedure to be actioned. If the pilots are unable to identify the correct ASI reading by comparison with the standby ASI then groundspeed can be used to calculate an approximate airspeed.

GNSS and INS/IRS systems have a groundspeed readout, this can be used to calculate TAS by applying the wind values from the computer flight plan. The TAS then has to be converted to IAS to find out which ASI is correct.  Not very practical for small discrepancies but better than nothing!