You can find that in the sysforeignkeys system table. Here's a query that will list foreign key info for the tables in a database:
select t.name as 'Table', c.name as 'Foreign_Key_Constraint', rt.name as 'Referenced_Table'
from (select distinct constid, fkeyid, rkeyid from sysforeignkeys) as FK
join sysobjects c on c.id = FK.constid
join sysobjects t on t.id = FK.fkeyid
join sysobjects rt on rt.id = FK.rkeyid
order by 1,2,3