Using social engineering techniques (LinkedIn, Facebook etc) it is feasible that a Data Subject Access Request (DSAR) may contain enough information for you to assume that the data request is genuine.
The valuable information you hold (particularly on ex-employees) could be used for identity theft or fraud. You might be called, as part of the request, to supply valuable items such as name, address, date and place of birth, national insurance number; all great ingredients of an identity thief’s toolkit.
If possible, seek to engage the requestor to both clarify the scope of the request and to ensure that you have sight of originals or certified copies of originals. Bear in mind that whilst identity verification may not be used to unduly delay a response, the DSAR clock does not start ticking until you have received the required information and may be required to prevent an inadvertent breach.