I planning on setting up the base near trade routes. But how can I check where I should build the base so caravans would visit it??

Theres a few ways:

you can stick your settlement between two cities (such as in the great desert), this would increase chances of passing trade / npcs etc, but you may also find you get a visit from the taxman.

you can check for visible signs of a road and place your settlement on it.

you can also use the in game... uh, thingy (i forget its actual name) to check for roads - i think its shift+ctrl+f11 - but be careful as making any changes and saving will permenantly change things in your install if i remember correctly (i only ever use it for auto bug fixing).

im sure somebody will have other (possibly better) suggestions.

Edit: thinking on it, actual caravans should attempt to visit anyways now, though i may be entirely wrong on that.

Well Im planning to make the Base in the swamp near Sharktown, since I wanna make the most powerfull drug cartel