For a practitioner this occurs when you feel or
hear the spirit calling to the person. In a custom conjuration only those who
wish to be companion to the person respond. For the Keeper they will know because
the spirit bonds with them.
It is important to work with a practitioner or seller you trust & you know has a good reputation in the field of Spirit Conjuration & Spirit Keeping because often it is the discretion of the practitioner to make the decision which spirit goes with which person. An experienced practitioner will make the best decision as their experience will be important in discerning the correct spirit for each Keeper.
As a Keeper when it comes to your decision in choosing a pre-bound spirit you should take into consideration the immediate reaction & gut instincts in reading or seeing the spirit. There is no substitute for the immediate, positive kinship you feel towards a spirit.