Flying Crane
Sr. Grandmaster
An experienced Karateka can tell you with authority whether or not you are performing a technique correctly. He or she can also critique your performance of a kata or any other aspect of your training within the specific form of Karate you train.
even there, I say it depends on who you trust for your information. There are a lot of people out there, in any system you care to name, who are wearing high rank and claiming mastery, and may even have a high reputation in the general martial arts world, who know nothing and are in no position to actually critique or instruct. Just the other day I was perusing Youtube and looking at videos of various people doing stuff from my system. Some of these people are teachers, one was a "grandmaster". What I saw horrified me, including what the "grandmaster" was doing. It was pure crap, based on the standards of my sifu. I wouldn't take any instruction from him, even if I was paid.
There simply are no objective standards that everyone recognizes. Everyone has their own interpretation of this stuff, some better and some worse, and some people who don't really know much can still be fairly effective simply because they are athletic and big and strong and they can take crude technique and simply overpower people with their aggression. But they are not actually all that skilled or knowledgeable about this stuff, but they believe they are and they can convince their students that they are. Make your choices wisely about who you accept instruction from because a whole lot of "masters" out there suck, plain and simple. And be careful about accepting critique from people who you don't yet know if you can trust their input or not.