The audience can tell. Shakespeare modernised the form of the sonnet, and transformed it from a stylised, courtly love shtick to a fluent and flexible form that could turn itself to any subject. This isn't to diminish the contribution of his forebears and contemporaries; but what distinguished Shakespeare from someone like, say, Sir John Davies, was the maturity of his means. None of this was accomplished by flailing "innovation", and this, I think, is the real poetic miracle of the sonnets. His strategy was twofold. First, he realised that human love was the one theme capacious enough to encompass every other - these are also poems about death, sex, politics, sin, time and space - and he needn't stray from its centre. Second, he did this with a minimum of experiment, writing the form into transparency, until it became as effortless as breathing.