If I have a union like

union Type
    struct Student s;
    struct Employee e;
and a struct person like

struct Person
    char firstname[10];
    char familyname[20];
    char telephone[10];
    union Type type;

how can i check whether type of union in person is student or employee?can i say this: if(p.type==s)

You can't, you need a separate field (usually of an enum type) to keep track of which field of the union is set.