If I did not think they would be a good teacher I also told them that. I said something to the effect that they could use me if they wished, but here is what I would say and I told them exactly what I thought of them as potential teachers. I assume they did not use my name.
Now on the other side of the coin, I am asked to write reference letters for teachers I have had when I was the principal and it is often a difficult task. After all, my integrity is on the line as well as the teacher's future livelihood. I have to walk a very fine line often but an experienced hiring person knows what to look for in a letter and can often determine the quality of the candidate from what is not said as much as what is said so I am not too worried about what I don't say.
If you are in mid career, they always need t see the name of your present principal but if that is not going to be so good, you can ask your department head to write something for you or some of your teaching colleagues. In fact, I would actually get them to write something ahead of time and keep a file of reference letters. that you can show or send if they are required. Please read what I said previously about resume writing, because if your resume is no good no amount of letters will help you.
If you are retired and you have no immediate supervisor ( other than your spouse!) you have a challenge. This is where you keep all of your notes and letters that you accumulated over your career and use them explaining that you are retired.
If you want to teach in Asia, the older you are the better it is because age is respected. If you look at TIE online, they actually tell you which countries there are age restrictions so this is a helpful web site.