I can tell you from the top of my head.

我想都不用想就能告诉你

I can tell you by heart.

我可以凭印象告诉你。

There you go!

你说的对！

I can pretty much understand what you are talking about.

我就想跟老师讲，我非常能了解你在说些什么。

I see／I got it.

我明白了。