How can one spot a jinn and subjugate it for his service, and what is the legal ruling on that, and can he bear that?

There is no known way to observe or control Jinn. The fundamental principle is that they are concealed from human sight, as stated in the Noble Qur'an: ﴿Indeed, he sees you, he and his tribe, from where you do not see them.﴾ [Al-A'raf: 27]. Jinn may be seen if they take on physical forms, as happened with the Prophet (peace be upon him) when an 'Ifrit from among the Jinn almost interrupted his prayer. As for the ruling on utilizing Jinn for those who are able to do so, it depends on the purpose of their utilization. If it is permissible, then it is allowed; if it is forbidden, then it is prohibited.