Having some difficulty understanding the best way to implement subclasses with a generic repository using Fluent NHibernate.

I have a base class and two subclasses, say:

public abstract class Person
{
    public virtual int PersonId { get; set; }
    public virtual string FirstName { get; set; }
    public virtual string LastName { get; set; }
}

public class Student : Person
{
    public virtual decimal GPA { get; set; }
}

public class Teacher : Person
{
    public virtual decimal Salary { get; set; }
}

My Mappings are as follows:

public class PersonMap : ClassMap { public PersonMap() { Table("Persons");

        Id(x => x.PersonId).GeneratedBy.Identity();

        Map(x => x.FirstName);
        Map(x => x.LastName);
    }
}


public class StudentMap : SubclassMap<Student>
{
    public StudentMap()
    {
        Table("Students");
        KeyColumn("PersonId");
        Map(x => x.GPA);
    }
}

public class TeacherMap : SubclassMap<Teacher>
{
    public TeacherMap()
    {
        Table("Teachers");
        KeyColumn("PersonId");
        Map(x => x.Salary);
    }
}

I use a generic repository to save/retreive/update the entities, and it works great--provided I'm working with Repository--where I already know that I'm working with students or working with teachers.

The problem I run into is this: What happens when I have an ID, and need to determine the TYPE of person? If a user comes to my site as PersonId = 23, how do I go about figuring out which type of person it is?