I think Ray is asking how to identify them once the dot has worn off on used tires.
In my pit box I have several colours of nail varnish (oh I can hear the jokes forming already ...) which I use to mark the inside vertical face of the wheel with when I mount tires. The reason I use nail varnish and not pen is;
1) stays on no matter what you clean the wheels with
2) gets automatically removed when you put the wheels in acetone to remove the tire glue
3) can still be seen easily at a glance even on dirty rims.
I just buy them every couple of years from the supermarket for about £1 a pot and they last ages as all you need is a dot! I keep green, yellow, pink & silver so total outlay is about the same as one decent paint pen.
Don't put it on the inside cylindrical face of the wheel as I find it can get rubbed off there.
Hope that suggestion helps mate.
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