With any vehicle, the center point of a turn will be in line with the back axle. If you have a tag axle, that's a little bit ill-defined, granted, but it'll probably end up being close to the main rear axle since it carries more of the weight.
A little thought will show you why this is the case. Each wheel, at any given moment in time, must be traveling in a direction perpendicular to its axle or else it's skidding to the side. When turning, assuming a constant turn (driving in a circular arc), all the wheels must be traveling tangentally to a circle centered at the center point of the circle you're traveling in--albeit at a somewhat different radius for each one--with the axis of each one perpendicular to that and so aligned with a radius of the circle. The front steer wheels of course don't each have their respective axis aligned with the front axle, but rather swiveled so that each one will intersect with a line extended from the rear axle at a single common point, the center point of the turn.
As others have said, how this works out in practical terms and how to judge things from the driver's seat is best worked out by practice and experience. There's no shame in getting out and looking, and doing so repeatedly if needed; it certainly beats failing to do so when you needed to.
