That rumor started because Wayne rapped Drake's verse from Money To Blow at the VMAs as a shout out to Drake before it was released. Now i think I'm Single was originally Drake's song but Tune liked it and took Drake's vocals and rewrote the song he probably kept the hook and used some of Drake's original cadence which explains why he has writing credit on it. And Gillie said himself that he never wrote for Wayne he just coached him on eastcoast lyricism.
It's fairly easy to find out who has ghostwriters when establishing a certain criteria and standards.
1. I'd start with comparing general pools of allusions and tropes made that make an emcee distinctive.
Adding onto that,
to how close the artistic relationship was from the protege rapper to their ghostwriter mentor.
If the ghostwriter's careers began way before they wrote for their understudies
, that is a dead giveaway.
If the references
are way too similar, have a certain attitude, use the same exact slang relative to that circle, etc. 
Then that isn't coincidence, they were given a "study sheet" based off of the ghostwriter's style OR they later bit off more than they couldn't chew up themselves being copycats.
Either way was the result of the ghostwriter's instruction to their roster, or someone they affiliated with in the industry. This is evident when comparing the protege's later material, upon realizing they suddenly do NOT write like that normally, which is either predictable, straight up generic, mostly wack/lazy, etc.
Ex. Compare Fat Joe doing "Lean Back" after Pun died, to later getting aired out that Pun wrote "Don Cortega" as an album, it makes complete sense logically.

When you remember how much he sucked total ass in light of everyone else on Terror Squad,
that fall off was a belly flop in writing standards.
Check out this article, I was shocked myself, Pun's wife is also still trying to sue him for using his name for things he never came up with himself.
10 Rappers Who Have Used Ghostwriters