I think it has a lot to do with experience. If you haven't been writing for a very long time you may not be as confident as you would if you've been at it a while and you've had enough constructive criticism to know which elements you're good at and which elements you're not so good at. For example, I don't think I have the greatest grammar in my songwriting (which for my style of writing isn't too bad of a con, but it really depends I guess) and I don't have a very excessive use of rich vocabulary. Occasionally I will use "big words" but only if I truly believe it fits and actually enhances the song...and if it actually makes sense. But I'm all about getting the point across, so I'm not going to bog a song down with rich vocabulary when my song could be a lot catchier if I just kept it simple. Other times, I may feel a little poetic and break out the thesaurus. But always let your feelings come first, then worry about grammar, vocabulary and stuff like that in your final drafts of the song. But on your rough draft let it be what you feel and you can make corrections later. This way you keep the emotion (which is a key element) in your song, even after it's revised, and you take the time to let the song sink in to where you can look back on it later and see what needs to be changed and what's okay as is (vocabulary, grammar, slightly better rhyme schemes if necessary). When you are for sure your song is in it's finished form (which can sometimes be a tough decision) you will feel a sense of pride and confidence from your hard work. You know all the key elements are there, and your emotion and effort still stands just as strong in your final work as it did in your rough draft. Over time, you'll get used to this feeling of confidence, and this overall method of songwriting in general and you will know when your work is good...and when it's not so good. Just remember there is always a time and place for everything.