Overdose - How do you know if your OD'ing?

[an_army_of_one]

Is there a way to tell if you are going to overdose by snorting too much coke in a night?

I know that your heart beats faster and one of the risks is hear attack but is there a telltale sign that means you need medical attention?

Suppose you are at a party and you have done too much what signs should your friends look for in the case you don't notice yourself?

Thanks so much and I hope that I am not rephrasing and asking the same question that has already been asked.