Is one who doubts the disbelief of a person or describes a believer as a disbeliever, whether seriously or jokingly, considered a disbeliever? And what is the ruling on saying "I am a sorcerer" jokingly when performing an extraordinary act?

The default assumption is that a Muslim remains a Muslim. Therefore, it is not permissible to declare someone an infidel based merely on doubt, nor to accuse them of disbelief without conclusive proof. Whoever accuses a Muslim of disbelief has committed a grave sin, but does not thereby become an infidel himself. It is not permissible to imitate sorcerers or to adopt this description. As for using the word "magic" (سحر) to describe an act that is extremely strange or causes astonishment, this is a permissible linguistic metaphor, and it is not forbidden. Indeed, it is mentioned in the Hadith: "Indeed, some eloquence is magic," and this is a praise for eloquence.