If you have alot of backlinks pointing to a new site, can you get flagged by google?
Will they notice and consider this a form of spam?

Yes especially if you get them all in a few days!
I spread out link building. Dont get 100's of links pointing to your site in a day or 2, google will really notice and punish the site.

Is that definite though?
HOw can you tell if google has penalised you for that?

I think you'll see your site drop out of the results pages.. thats how you know.
And you can contact them.

I hear they let you back in sometimes too.

The way to tell is that you will not find your site popping up in search results. Once you are banned for this and other spamming or inappropriate link or site activity, Google is not likely to consider you again.

I think that will be noticed so have a plan from some seo expert to flag your site .
Only backlink cannot support a lot you have to do something else too .

It's just a mechanism to keep Google from being vandalized, and keep the searches relevant. Normal sites shouldn't get a hundred backlinks in a day after receiving none the previous day.