The first, and possibly the most important part, of critical thinking is to recognize that you can't be right all the time, and embrace the instances where you're wrong as learning opportunities. I am glad to see that you seem to be pretty good at that already :)

Aside from that, I don't actually have anything concrete for you, unfortunately. What's worked for me is reflecting on my own biases and confidence in the information I am bringing to the conversation. In your case here it's clear that you started your comment with "I think this person is fake" and constructed a (tenuous) chain forwards to arrive there using assumptions rather than concrete information. We all do this to some extent, but specifically taking time to look for this kind of thing can help reduce the chances of it happening. Another skill you can learn (generally, by interacting with people you disagree with) is the ability to run your own devil's advocate on your comments. It sounds a bit strange to say it, but a lot of what I write gets much stronger pushback from myself before I even send it than it does once it's out for others to respond to.

As for practice, you can do this anytime you interact with anyone. As long as you are interacting in good faith, an open mind, and with genuine curiosity, people are unlikely to ban you. What you might want to keep in mind, however, is the context surrounding the conversation: getting something wrong about Java is regrettable, sure, but ultimately not a big deal. But outright calling someone a fraud is a pretty serious accusation, especially considering that certain groups of people are often more affected by this problem. When talking about real-life people, you should be very careful about the conclusions you draw and what their consequences may be.