The strength of a magnet can be determined by measuring its flux density (B) which is expressed in teslas. The flux density will vary according to where relative to the magnet it is being measured. The instrument for doing this is a flux-density meter (which was called a 'gaussmeter' - 'gauss' being an obsolete unit of measurement for flux density, from the cgsA metric system).
At a given location, you could measure the strength of the field in both the horizontal and vertical directions. From this you may determine the azimuth of the field at your location, its polarity, and its dip angle. In addition, the field will vary in strength throughout the day. And if the Earth is subject to a 'solar storm' at that time, the variability may well be rapid.
There are no special types of magnetic field - the shape and strength of the field may vary, but the fundamental quality of the field is the same. The reason why stainless steel is not picked up by a magnet is that stainless steels are mostly not ferromagnetic, and therefore have no strong interaction with a magnetic field. Although paramagnetic and diamagnetic materials do interact with this field, it is very weak; nowhere near enough…
You said "armature" so it is a dc motor. Hence if the field is permanent magnet type then a voltage appears at the armature terminals nd its magnitude depends on the speed nd magnetic field strength. If it's field coils, then they must be seperately excited (if it don't possess residual). By changing the field strength you can vary the voltage produced at armature terminals.