How do they know? Mainly it came down to how the different Elisabethan authors used language. Experts specified their linguistic fingerprints. ( their way with words, phrases, rhythms and structure of their sentences). Astonishingly higly congruent contextual contents between Shakespeare and those authors were of little or no interest.
The "bizarre" result in this chart clarifies who wrote which bits of which plays: 89% of the canon was by William Shakespeare, and ten contributions by other playwrights account for the other 11%.
Not all of the collaborations are said to have been operated the same way. Sometimes, as with „Titus Andronicus“ and „Pericles, Prince of Tyre“, experts speculate Shakespeare took over plays that had been planned and started by other writers, and made them his own. Sometimes experts speculate that Shakespeare adapted a play that was already finished: such as "Henry VI, Part 1" (supposedly written by T.Nashe and T.Kyd??) (originally entitled „The Contention between York and Lancaster“, and now known as Parts 2 and 3 of Henry VI; it was later acquired by Shakespeare’s company, whereupon Shakespeare turned all three plays into a serial and wrote three new scenes for Part 1. Sometimes his own plays were likewise adapted: the non-Shakespearian material in „Measure for Measure“ and „Macbeth“ is believed to be added after his death by Thomas Middleton etc..
Experts also speculate that Shakespeare may have worked for his company, the Lord Chamberlain’s Men, not only as actor and principal playwright but also as something akin to a head writer or script editor (Martin Wiggins) . Such traces of activity came during a period when Shakespeare‘s own output of solo-written plays is beleived to have fallen to one per year, This would explain his intermittent interventions in "Sir Thomas More", and he may also have contributed to Ben Jonson’s Roman tragedy, „Sejanus“ (1603), written at the same time as „Measure for Measure“; Jonson later acknowledged that there had been a second author – whom he said he respected but did not name – but carefully revised out the man’s work before publishing the play in 1605.
Because no one to this day can imagine and has systematically checked , wether most of the supposed co-authors (such as Peele, Munday, Heywood, Dekker, Middleton, Fletcher ,Wilkins, Chettle [and others]) were real pseudonyms or taken as pennames from living and/or deceased figures by the true author, alias Marlowe [read Summary]