Credit card numbers stolen. How to check if my MacBook has a keylogger installed?
The short answer is you already have! 

Malwarebytes, like most (if not all) antivirus software for Macs, also guards against other nefarious activities, including keyloggers.

The free trial of Malwarebytes (which I assume is what you used to scan your Mac), would have identified any threats and cleaned your Mac of any infections. The free trial also comes with 30 days access to their premium service, so you could also take advantage of that as well.

Assuming you're a personal/home user, then many of the free options will most likely meet your needs, so don't feel obligated to keep using Malwarebytes if your preference is otherwise. 

There are many other options out there you can choose from. As a starting point you can refer to the AV Comparatives list of antivirus vendors for Mac.

You may also want to read the most recent (as of now) AV Comparatives Mac Security Test and Review report (conducted in 2017).