In no particular order………..
1. You are able to sing when you are not in perfect condition…such as having a viral cold.
2. You are good without having to work very hard at it.
3. You can express yourself with good pitch, diction, volume control and phrasing.
4. You can speak with other singers and musicians about “the lingo”. You can refer to keys, passages, beat, rhythm, tempo, and more.
5. You can perform at your best in any venue regardless of size, location, audience or conditions.
6. You know how to practise and rehearse to be ready.
7. You can roll with the punches. In other words, you are able to adapt to less-than-perfect conditions such as a guitar being out of tune, or a monitor feeding back.
8. You know how to work with, and respect other musicians. You are prompt, well-rehearsed, and ready to go. You know how to dress appropriately, provide your own equipment and have music details ready for accompanying musicians.
9. You do not waste other people’s/musicians/singers time.
10. You get asked back.

That’s just the top ten that came to mind in the last 10 minutes…………I’m sure there are more….just write them in the comment box if you think of any.