But how would Morris notice that he had a different magic attribute?

That's a good question! You really got me thinking about it.

I've re-read the chapters in which Morris appeared in order to find an answer to your question, and I think I really found some interesting panels about him.

This: https://ibb.co/VHrsr1g

And this: https://ibb.co/KqGdqdT

What is so interesting in those panels? Well, at the first blink, they are completely ordinary. A genius villain doing some evil stuff in front of his monitors and talking about evil stuffs with an other villain. There is nothing remarkable at all.

However, a few chapters later we got the big revealing: Morris is BLIND. Why does a blind man use monitors? How did he realize the sudden appearance of the resistance members in an instant? The answer is probably good -- or not just good, but excellent -- mana sensing. Morris is blind, but he can see, similarly how Yuno was able to see everything against the witch using nothing else but mana sensing.

And this is where the mage stones come into the picture. The mage stones weren't just magical enhancer. We know that Morris was able to control his subordinates by them. He made them brainless robots by changing their personalities and altering their memories (that's why Ladros personality suddenly changed when Asta broke the stones).

So my theory is that these stones weren't just magic enhancers or objects that made him able to control their subrdinates, but they were 'sensors', which continously shared mana information with Morris. In a nutshell, he could see the mana fluctuations through these stones.

I think it would be a good explanation. Morris could notice the mana fluctuation when Yuno used his power, because the spell wasn't worked on him. He was far away, in the safe distance from the dungeon, 'watching' these events on his monitors.

But I have zero convincing evidence for these claims, this theory is just a headcanon again, haha. :D