You can search Google, and find many "Google Sandbox Checkers", but they don't actually work. You'll know when you've done some heavy backlinking and your site is no where to be found.
Here's a checklist, as Swiftly said the sandbox checker tools don't work. You need to use your initiative. One of the most common myths is that an entire domain is sandboxed. It isn't sandboxing works on a per keyphrase basis --- therefore you may be ranking for longtails but not your competitive phrases. Check out my Google Sandbox Checklist
If you search your URL and show up, you are not deindexed. If you disappeared, ie from page 1-3 to who knows where for your primary search term, you are probably incurring some type of penalty. Been there done that, for a one of my site. I wws lucky to only incure a 30 day penalty. Even afte rthe penalty expired, I put in a request for reconsideration. The request was worth it.
If Google sees that you have built a large number of links in a very very small span of time then it has a special place for your site which it calls the Sandbox. It will place your site there for 3-5 months while at the same time looking at your site for any change in links and traffic. If it seems fit you will go back in the rankings race, otherwise... lets just say you will have to start all over again.