Sandbox doesn't mean that your site isn't indexed. It is basically a hypothetical filter that restricts you from ranking well on competitive terms. There is no way to check if you're sandboxed. However if you're Seo is good enough to have you ranked for a keyword and your site is fairly new and you aren't ranked for that keyword then you can assume that your site has been sandboxed.
If your site jas been into the sandbox, there are no certain sign to tell you. All what you can do is keep updating your site with original content all the time.
and maybe not sandbox.. since google didn't mention anything about it... if your site is not index for a long time then maybe its ban or might miss something like disabling bots to index your site..
If for some time, google is not included on your website page, there may be the beginning of the sand box
Post quality content... 1. Ping 2.Submit Rss Feeds 3. Social Bookmark That will get you going.. Best Wishes