For any artist or song you've ever liked, there has only been one true test of whether that person is a great vocalist. Or whether the person has talent or not for that matter.
If you're a recording artist/singer/vocalist, any song you create goes through editing and looping to make it better. All the music we hear today has been edited, enhanced, cut in some way. Do you know why an artist (esp. in the US) doesn't sound like their album released versions? It's because they suck and they are not up to par. The producer or sound editor has to improve the song and make it marketable. If you, as an artist cannot sound like your album-released "single" then you are sub par. In fact, you lack the normal talent to make the song what it is. A producer has to improve the song electronically to make it appealing to consumers.
That's why when you perform live, its always different than the album version. It isn't because you want to spin it a different way or give it a unique style for each and every live performance. It's because you naturally cannot sing the song so well or like the album version. The producer edited it irregardless of whether you can sing it that well or not. His job is to make it pop and sound great; eg make it marketable. Your innate talent is most likely lacking.
That's the test.
If you can perform a song live, acoustic, "stripped" and have it sound as good or better than the album version, then you are good.
If you cannot sing your own song like the album version, you are sub-par and in fact suck. The fact that you weren't able to sing the song as it was originally released means they had to go to a far extent to edit it and make it better for sale. You as an artist, then, cannot match the "master" version of your own song.
Logic and reasoning dictate this.