How do you know how quick you can take a corner?

[TheBigDawg]

Now I know that on straights, you can theoretically go a limitless speed, and you won't spin out.

But when it comes to corners, with Newton's Laws of Motion & Centripetal Forces, etc... you can only go so quick, before your tyres will lose grip.

So I'm wondering: how do you know how quick your car can take a corner?
What do you use as a reference to work how fast you can go, and where the limit is?
(or at least, when you're getting close to the limit!)

Thanks for any responses.