You can install a program called vnstat to monitor your droplet's traffic usage, see Where can I see the used traffic of my VPS? | DigitalOcean.

The 1TB limit isn't an actual limit, your droplet will not lose connectivity if you pass that limit. However, once we introduce bandwidth billing, you will be charged $0.02 per extra GB of traffic. We are not charging for bandwidth right now. :)

https://www.digitalocean.com/community/questions/where-can-i-see-the-used-traffic-of-my-vps?answer=1130