The MCA recommends that the manual steering be tested at least once a day and before entering areas where caution is required. During this test, the wheel (or equivalent) should be engaged and the ship steered by hand.
Only by checking with the employer, who may be able to check with the issuing Administration, since the Administration is required under STCW to maintain a register of certificates issued. (The MCA does not keep records of non-UK certificates, and has no means of verifying their authenticity. )
No. He will be bound to serve for a further period of 42 days from the date of the ship's departure from the UK before being able to give 7 days' notice as in the last answer, provided the minimum 3 month period has expired.