There isn’t really a way for you to judge your own skillz (as all the terrible singers on American Idol will show you). I think the best way is to show somebody who you trust and ask what they think – but if you’re too shy to do that, you could always post some videos/audios of yourself on the internet, ideally in some music-specific communities (e.g. the Singers community on LiveJournal), and ask for opinions there.
You could also see a professional vocal coach/singing teacher and get their opinion, as well as their guidance.