Could you discern a good teacher from a bad teacher when you were in school and the topics were new? Did you ever find out someone was unreliable after they gave you some facts and later you discovered they were wrong? You won't have a 100% rate of detection and it'll be lower than in subjects you know about, but after a few times you can see if the new stuff it tells you is right or not.

Because it can be easy to test the result even if you don't know how to come up with a solution ?

If I asked you to build a light switch I don't need to know anything about EE to see that the light turns on/off when I flip the switch.

LLMs often fail to provide functional solutions for non trivial stuff (not impressive) - and once you know more about the domain, you usually figure out that the light switch will likely burn down your house in few hours because it's overheating.