Is there a way to tell if you've completed the magnum opus?

[Unruly Ruler]

???

 

[FancyMancy]

If you were employed as the Manager of something, or if you were the CEO of something, you know you wouldn't need to ask.

 

[Head Guardian Blitzkreig]

???

Yes of course. You will know it is done when your nose starts bleeding, your hair turns purple, and your fingernails fall off. Then you fast for 40 days to finalize the process, calling the ambulance on the last day.

All jokes aside, you will know during the entire process. One can tell the difference just in opening a chakra, let alone completing the whole magnum opus. You will also be working with a demon through this and long beforehand.
To put it lightly, the MO is a huge deal. You will be aware of all parts of the process.

 

[Henu the Great]

When you are nearing it's completion such questions are long gone being relevant, so don't worry and follow the 8 fold path.

 

[Aquarius]

You're gonna be exceptionally advanced and you will be able to tell.
If you ask you're most likely far far away from it.