Software to determine a particular NOTE from a riff?
Is there such a thing?
Melodyne, maybe??? 
Melodyne can do that. Ear training is more reliable but harder.
I bet youve got a midi keyboard. Hit notes until you find the root note.
Youll be amazed at how little time this takes to learn, and how vastly useful it is in your DJing.
I sold my midi keyboard to a user on this forum…
I’m a little worried that I’ll be using a full software solution (cummulation of errors!) where I’ll use MiK to find the KEY (and therefor, rule OUT some notes) then melodyne to find the NOTE…
Classic. 
We both got a great deal - he even collected it from me! 
I’ll be using Ableton, so I’ve got no problem using a QWERTY keyboard for playing individual notes…
Use spectrum on the track and look at the fundamental frequency. Hover over it and it will give you a note