Software is cool, for some things it's accurate. I find I'm a lot faster than software as I can hear the song and know the key in seconds. The video above is good. There will be times where won't be able to pull up your app especially if you work in and out of studios.
This, or if you have FL studio, go into the editor<click on the flag< detect pitch regions
from there you can just go to the tool tab in the sampler and choose the key it's in.
and what happens if you don't have FL studio or the studio you're in doesn't have it? Are you going to make the client wait you try to find an app that does it. Train your ear, it's a great skill