Now i am going to try some long exposure photography. For example i want to shoot the skyline in night where the buildings are well lit but still get a night feeling. How do i determine what shutter speed i should use ?

You can continue to use the trial and error method you've been using, and that's fine, but with long exposures each test shot can take a lot of time.

Because you can trade ISO, aperture, and light for shutter speed, you can do some test shots at high ISO and large aperture to get the exposure right and then adjust those parameters and compensate with a longer exposure. For example, if a test shot at 1 second and ISO 3200 gives the right exposure, you can decrease ISO to 100 (that's 5 stops) and increase the exposure to 30 seconds (5 stops). If you don't want to mess with ISO, you can change the aperture instead, or add an appropriate neutral density filter, or some combination of all three.

It's a simple idea: for every stop that you increase exposure length, you need to compensate in the other direction with some other parameter.