There isn't a simple thing you can buy to do this.
If you think about the problem - (it's the force on a loudspeaker voice coil per amp of current through it) then you would need to know the length of the coil and the mean flux density.
No simple instrument can measure these.
You can measure it statically with some simple equipment (on bass and mid range speakers, not so easy on tweeters).
You need a way of accurately measuring the position of the speaker cone (a dial gauge will be fine but you could use reflective tape and a laser pointer or even a microscope).
Put a non magnetic weight on the cone - ideally an annular shape so that the cone deflects as much as you feel safe with.
Apply current to the coil until the cone comes back to its original position.
You know the weight, gravity, and the current - so you can work out the force per amp.
There is another way with many descriptions on the web:
