It is a superstition.
You write the name of a person you like on the bottom of your shoe. If the name fades away fast, the person does not have the same feelings. If it stays for a long time, that person likes you.
In this case, the sentence you quote means that the person mentioned does not have the courage to really find out if the person he/she likes feels the same way, so the image is that he/she can only use some superstition to find out.