When buying telephoto lens adapters, BEWARE that certain lenses labelled as  2x  may be only around  1.4x  or  1.5x , and lenses labelled as  3x  may in fact be only around  1.7x  or  1.8x .

In order to tell the magnification factor, measure an object in a "telephoto" shot, measure the same object in a "normal" shot, then divide these two lengths to get the x0.nn factor.  For more accuracy, repeat this, and use as large an image as possible.

Example, an object measures 67mm in the "telephoto" shot, 45mm in the "normal" shot; 67/45 = 1.49x.

Example 2, an object measures 6.9" in the "telephoto" shot, 3.3" in the "normal" shot; 6.9/3.3 = 2.09x.

IF A LENS DOESN'T HAVE ACCOMPANYING PICTURES taken with the actual lens, DON'T BUY IT ! 

If it does, make sure they are taken with the ACTUAL LENS, and not generic photos off the internet.  Or off the lens box itself - the one I've got is inaccurate.  The "telephoto" shot is merely a cropped, smaller version of the "normal" shot.