If your map has decent access to fresh water then a little planning should mean you will need few aqueducts in your core cities.
Sometimes it can be hard to tell if a particular tile is actually bordering a river or fresh water. A few hints that can help you tell -
1. If a tile is next to a river, it will automatically have +1 gold per turn (gpt) added to the square. Most squares have 0 gpt base value, so if you see a square near a river that has 1gpt base value, then it is touched by the river had has access to fresh water. Luxuries/resources may throw this off, so be careful on that, though you would likely not settle on that square anyway, even if it has access to fresh water.
2. An inland fresh water sea/lake will have 2 food and 2 gold as a base (salt water has only 1 food and 2 gold).
Not sure if anyone lurking knows how this works - I don't recall if I have ever tested it - if the fresh water lake is within the 8 immediate working tiles of the city, does that count? I'm thinking about if the fresh water touches the City at a corner rather than the flat of the city square. That might be risky. Have to test that. But if it touches a flat side of the city square then you are good for sure.