The problem is that you really can't tell. You know if you get on with the person. You know if they're interested in you and prepared to spend time with you. But I look back now at my first instructor for my PPL(H). He seemed to be all of those things. It took me a lot of hours to realise that he didn't know a lot about instructing, and really wasn't teaching me very well. He may have been a nice guy, but he didn't know that much about flying helicopters, never mind passing it on. Now, as a wannabe instructor myself, I have a certain amount of sympathy for his difficulties; teaching people to fly helicopters must sometimes feel like an interesting way to commit suicide! And he didn't have that many more hours than I've got now. But at the time, when I realised I wanted to change instructors, I was angry, and also blamed myself for being taken in by him. But how could I have known? The answer is that I couldn't. That's why it's a good idea to fly with someone more experienced like the CFI once in a while. It was during my solo check ride that I realised things weren't going as well as I'd thought, and I think everyone else did too. But it shouldn't have had to take that long.