A site of mine that has existed for about 7 months finally started ranking in Google for some of it's keywords.. Although rankings aren't too high as of yet (top 200,300) I jumped into these spots from absolutely nowhere.. so I'm happy =)
you will know if you have google analytics installed. You will see all the keywords people are using to find you. If one day you see ZERO hits from google search (and lasts for several weeks), I'd assume your were being sandboxed.