The city should be adjacent to the fresh water. Before you place the city, right click on the tile and see if it has a gold piece available on it. That gold piece would indicate access to fresh water. No gold, no access. That is unless the tile has gold from another source, such as a resource of some kind. Then there would be gold for the resource plus an extra gold for the fresh water access.
In Despotism, the standard tile penalty may negate the extra gold in some instances, so occaisionally you just have to take the risk. Some are pretty easy to tell just by looking, but I still check on some.
And of course there are aqueducts, as Quintillus said.