Many people already know the service Have I Been Pwned (HIBP). There you can enter your e-mail address (or somebody else's), and discover if your e-mail has been found in any data leaks on the Internet. Leak in this context means a service where you have previously registered yourself, the service got hacked and data about you have been stolen and leaked online.
What most people don't know is that there are similar services out there, with German HPI & their Identity Leak Checker has an even bigger database than HIBP. Here you can also enter your e-mail adress, and receive an e-mail with an overview of any leaks where you appeared.
When I checked my personal e-mail address, I found myself in 6 different leaks, where 5 of them also includes hackers probably gaining access to my personal password for those specific services.
My results looked like this:
Please do check yourself on both services. HPI has a stricter focus on privacy, so you cannot look up information about others, only your own e-mail addresses controlled by you.
HPI also has some interesting statistics, a FAQ, as well as information about the e-mail you receive from them.
If you find yourself in any such leaks, don't panic. Some of these leaks are data collected from other leaks, and can be many years old, with names unknown to you. HPI has also chosen to make it so that you can only check information about yourself, using e-mail addresses you are in control of now.
The most important thing for you to do is to make sure you've changed your password at any listed services after the date provided in the e-mail you receive. Furthermmore you really shouldn't use the same password across multiple services.
Checking yourself can be smart to do a couple of times per year.