Refer to figure. 
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.'

Airspeed indications can become unreliable because of erroneous data from pitot/static systems, AoA sensors or air data computers. Depending on the size of the error, fault detection systems can sometimes alert the pilots but this isn't always the case so pilots need to be aware of the means by which it can be recognised. The learning objective lists the following:

• Differing airspeed indications between ASI's - airspeed indications are shown both PFD's and a standby ASI.
• Unexpected aircraft behaviour - aircraft handling changes with airspeed, do the handling characteristics match the indicated speed?
• Buffeting - exceeding high and low speed buffet boundaries will cause buffet.
• Aircraft systems warning - Boeing aircraft display 'IAS disagree' on the PFD if airspeeds differ and Airbus aircraft may display ADR (air data reference) alerts.
• Aircraft Attitude - a high nose attitude when not at a low indicated airspeed would be unusual, attitude is shown on the PFD.

1. Flight controls vibrations → INCORRECT.  Unlikely on CAT aircraft, which have powered flying controls, as there's no feedback of aerodynamic buffet.

2. Floor vibrations → INCORRECT.  More likely to be caused by mechanical equipment under the flight deck floor than excessive or very low airspeed.

3. Message on the PFD → CORRECT.  For example on a Boeing 737 an IAS disagree alert is shown on the PFD if the indications differ by more than 5 kts

4. Different airspeed indications on the ASIs → CORRECT.  This is the most obvious indication.

5. Buffeting → CORRECT.  High speed or pre stall buffet which isn't reflected by the indicated airspeed.