A trusted virtual private network is a great tool for security and privacy, but if it's not configured correctly it may not be so private.
A trustworthy virtual private network (VPN) is a good way to keep your internet usage secure and private whether at home or on public Wi-Fi. But just how private is your activity over a VPN? In other words, how do you know if the VPN is doing its job or if you’re unwittingly leaking information to prying eyes?
To find out, you first need to know what your computer looks like to the internet without a VPN running. Start by searching for what is my IP on Google. At the top of the search results, Google will report back your current public Internet Protocol (IP) address. That’s a good place to start, but there is more to your internet connection and its potential for leaks.
Diving deeper
An example analysis from IPLeak.net.
Your public IP address is one way private information can leak over a VPN, but you can also leak information via Domain Name System (DNS) queries, WebRTC, torrents, and geolocation. To see what you look like in your default state, visit IPLeak.net. This website checks all the previously mentioned methods for leaking data. Take note of all the data you see on this page so you can compare it to your VPN’s.
Now close this site, connect to your VPN, and navigate back to IPLeak once again to see what you look like over your VPN of choice.
Not all of these tests happen automatically. The torrent test, for example, requires a small torrent file (available via magnet link) to run before it can detect any potential leakage.

The geolocation test is helpful, but keeping your location secure is pretty straightforward. Just don’t allow any website to use your location while on a VPN. One way to do that is to specify a browser, Firefox for example, as your VPN-only browser. Then disallow location requests on that browser. Alternatively, you could use a browser extension that provides a fake location to websites that request it.

The most likely culprit for leaked information, however, is DNS. To navigate the web your machine requires contact with DNS servers to help translate website addresses from names to numeric IP addresses. Typically your PC automatically uses the DNS servers of your internet service provider. The problem is that if you’re using a VPN and leaking DNS through a local service provider, you can reveal enough information to point anyone spying on you in the right direction. That’s why VPN services often funnel their customers’ queries through DNS servers that aren’t connected to your ISP.
http://www.ttechbay.com/2018/06/09/how-to-check-if-your-vpn-leaks/