If current produces a magnetic field, how can the current be known?
External magnetic fields can in theory affect the current in a wire. This is well used in transformers. This is because in any place where the magnetic field changes, there's automatically an electric field which affects the motion of electrons. That's the induction.

But if you want to measure magnetically an existing current in a wire (=you want to know what the current is, no matter what fields have caused it) you need to put the wire through a current clamp. The clamp takes into the account only that magnetic field which is circularly around the wire. External fieds do not affect the measurement, altough they can have some effect to the actual current.