If you are using ASP.Net Membership then you will have table aspnet_UsersInRoles
which has the UserId and the RoleId mapping. From here you can determine whether a particular User belongs to a Role or not
select * from aspnet_Roles
inner join aspnet_UsersInRoles on aspnet_Roles.RoleId = aspnet_UsersInRoles.RoleId
where RoleName = 'Administrator' and UserId = '5E25F30B-C696-4321-8FB1-B6E2571A0A39'