There are two "magnifications" which are related to addon lenses: the maximum magnification achievable in combination with a given camera and relative (optical) magnification.
To know maximum magnification (ratio between object size and image size at focal plane) you need to photograph a ruler parralel to the frame as close as possible and then divide the viewable distance by sensor width (search for it online). This will give you absolute magnification. This ratio is typically 1:1 (object:image) for specialised objectives for interchangeable cameras and may be few times more for general purpose objectives. It does not characterize the recorded object size well though - 1:1 magnification with small sensor will give magnitude larger digital image of object than big sensor camera with same magnification would.
To know the relative magnification, you need to photograph a ruler from same point both with and without the addon lenses (ideally: with fixed focusing distance). Dividing ruler distance visible without the addon by distance visible with addon attached will give you roughly the optical magnification.
The only thing that i don't understand is the difference between the focal length and the distance between the object and the lens? I thought the focal length was the distance between object and lens.
Focal length noted on photographic objectives is more complex than optical focal length. Optical focal length is distance from the lens at which parallel rays converge. Photographic focal length (aka rear focal length) is a distance from rear nodal point to the sensor.
You may know optical focal length roughly if you project the sun (ideally: the bunch of parallel rays) into the sharp spot onto some non-burning material and measure the distance between exit lens and the sun image.