A Conjurer knows a spirit wants to be with someone because the Conjurer feels or hears spirits calling to the person. In a custom conjuration, a spirit who wants companionship with the person will respond.
A Spirit Keeper knows a spirit wants to be with them because the spirit companion interacts, communicates, offers support (even if it’s subtle to the Keeper at first), visits in dreams, offers telepathic messages or intuitive impressions, and any other form of support the spirit companion can bring.
It is crucial to work only with a trusted Conjurer who is well experienced – specifically with Spirit Conjuration related to Spirit Keeping. Ultimately it is the Conjurer who decides which spirit goes with which person. An experienced, trustworthy Conjurer will invest whatever time and effort is necessary to best make that decision for you.