Fatih, in reagrds to good building soil, have you come across some past posts that talk about the "bucket test"? This might give you a way to do a test at each site you look at. It would take time for each test per site to know if it is better soil ( and it still may not gaurantee it good), but it could instantly let you know if the soil is terrible for a pond. And of course, the ground deeper can be totaly different than whats on the surface.
No matter what, I would still have a profesional give you advice on the soil once you find a "likely" spot.
