When testing accounts for automation, a metric @ZellaQuixote and I have been using is whether the accounts A) post 24/7 and/or B) post 90% or more of their tweets via automation apps/services. We decided to apply this test to our own followers.
Of 32045 accounts that follow one or both of us, 232 tested positive for automation per this test. This chart shows the 25 most common apps/services used to post the tweets. IFTTT and Buffer are the most prominent.