Before you go off and say someone is talentless, I'd like you to remember something:
Testing for talent is like testing for drugs.
Now what do I mean by that?
Let's say three friends each took some drugs at a concert. One kid only took a little bit, another took a decent amount and yet another took an absurd amount. On their way home their car smashes into a tree. A policeman comes and takes them to the station for a drug test. Each kid took a different amount of drugs yet their tests will each come back with the same result—positive.
It's the same thing with measuring talent.
I suppose people can be one of three things in life: a minor talent, a major talent or not a talent at all. Only one of them will fail the talent drug test and that is the one who has no talent at all. Everyone else passes—even if their talent is as small as Tinkerbell.
One of my biggest pet peeves is when someone incorrectly refers to a musician as talentless. Chances are if you've heard of the artist, they're more than likely talented. It doesn't mean they're as talented as Bob Dylan or Paul Simon. It just means they passed the test.
Most people do not pass the talent drug test. How do you know for sure when someone does? You're going to hate this answer. You know someone is talented when everybody is listening to and watching them. People simply do not engage with artists that lack talent.
Now this has nothing to do with how good someone is. We all have a different test for that.