At the briefing stage the ATO should be making it clear that the pilot under check is to make all COMMAND decisions. If the ATO has to intervene in any way it is usually (should be) a FAIL. Normally one can't fail while receiving dual instruction, but the necessity to instruct on a test is fail material of itself. Depending on the ATO, a test gone wrong is sometimes allowed to continue on the basis that it is then training and would then be logged as ICUS or dual.
The only tests I ever logged as ICUS were initial rating or endorsement tests, because of course at that stage I was not qualified to be in command under the IFR, or in command of the particular type etc. But certainly not dual because the 'instruction' was over, or I would not have been there in the first place. Ditto with line training - the training logged as ICUS but the check-to-line as command. The nature of the check and name of the check pilot/examiner always clearly shown in the notes alongside the flight, so anyone auditing the logbooks would know that there had been no attempt to falsify the entry or over-claim command hours.
Taking cover from the experts now.......