I don’t think DO has implemented the charge yet.

As of now there is no way to know you exceeding the limit but DO has clarified previously on forum posts that before they start charging for excess bandwidth than allocated, its free till then for all users. They have also stated that by that time, the control panel would have provision to show volume based usage report also.

For the time being for your own reference and understanding you can install tools like vnstat and use it to monitor your usage from your droplet !!

Thanks..