Halliday & Matthiessen (2014: 282-3):
With a verb other than be it is clear which is Token and which is Value, since … this can be determined by the voice: if the clause is ‘operative’, the Subject is Token, whereas if the clause is ‘receptive’, the Subject is Value. … With the verb be one cannot tell whether the clause is ‘operative’ or ‘receptive’; the best strategy for analysing these is to substitute some other verb, such as represent, and see which voice is chosen.